6cups(750 g) powdered sugarmeasured and then sifted
1teaspoon(4.2 g) vanilla extract
1/3cup(77 g) heavy whipping cream
For the Cake
Preheat the oven to 350 degrees F. Prepare three 8-inch or four 6-inch round cake pans with non-stick spray, parchment paper on the bottom of the pan, and another coat of spray. Set aside.
In a medium sauce pan, cook the butter over medium heat. There's no need to stir. Let the butter melt and then cook. It will boil and then start to foam a little. Once you see the foamy layer, swirl the butter so you can see if it's turned a light brown and has brown specks in it. You'll be able to smell the brown butter as well. At this point, pour it into a liquid measuring cup and let it cool. You can do this step ahead of time and refrigerate the butter until you're ready to use it.
In a medium size bowl, whisk together the cake flour, baking powder and salt. Set aside.
In the bowl of a stand mixer, fitted with the paddle attachment, beat the brown butter, oil, and sugar on medium-high for about 3 minutes.
Add the eggs and the egg whites, one at a time, to the bowl and beat on medium until well incorporated.
Turn the mixer on low and add 1/3 of the flour mixture to the butter mixture and mix until fully incorporated. Then add half of the buttermilk and blend well, followed by another 1/3 of the flour mixture, the second half of the buttermilk and lastly, add the remaining flour mixture and mix until incorporated.
Add the vanilla extract and beat on low until incorporated. Scrape down the sides of the bowl and mix on low for another 30 seconds.
Evenly distribute the cake batter among the three 8-inch pans, about 16 ounces in each or 12 ounces in each 6-inch pan, and bake for 20 to 25 minutes, until toothpick comes out with just a few moist crumbs on it.
Remove cakes from the oven and let cool in pans for 10 minutes before inverting onto cooling racks.
When cakes are completely cooled, I like to wrap each cake layer individually with plastic wrap and stick it in the refrigerator or freezer until I’m ready to frost. The cake will be easier to work with if it’s cooled a bit. If I’m making the cake a day or two before I actually need to frost it, I put it in the freezer to ensure freshness. If I’m frosting same-day, I’ll just put it in the refrigerator to chill until needed.
For the Marshmallow Fluff
In the bowl of a standing mixer, whisk the eggs and salt until fluffy and frothy.
Meanwhile, heat the sugar, corn syrup, water and vanilla extract. Heat over medium heat until the sugar is dissolved and just simmering, about 5 minutes.
With the mixer on medium speed, add a tiny bit of the hot sugar mixture to the egg whites at a time. (Temper the eggs - don't scramble the them)
Eventually, all of the sugar will be added and then turn the mixer up to high. Beat the mixture on high for about 5 minutes or until very stiff and shiny.
Best used right away.
For the Rice Krispie Treats
This step can be made ahead of time, but should not be added to the cake layers until the day you plan to serve the cake. The Rice Krispie Treats will soften and lose their crunch if you assemble this cake too early.
Spray a 9x13" pan with nonstick spray. Set aside.
In a microwave safe bowl, combine the butter and marshmallows and microwave for about one minute. You're looking for the marshmallows to get big and fluffy.
Stir the marshmallows and butter well.
Gradually add the cereal and stir to evenly coat the cereal with the marshmallows.
Pour and pat the cereal into the 9x13" pan. Let sit for several hours to harden.
For the Frosting
In a medium saucepan, cook only 1 cup of the butter over medium heat. There's no need to stir. Let the butter melt and then cook. It will boil and then start to foam a little. Once you see the foamy layer, swirl the butter so you can see if it's turned a light brown and has brown specks in it. You'll be able to smell the brown butter as well. You'll want to do this step ahead of time and refrigerate the butter to get it back to a solid, cold consistency before you use it in the frosting.
In the bowl of a stand mixer fitted with the paddle attachment cream the one cup brown butter and the one cup of regular butter on medium speed for about 3 minutes, until the mixture is light and fluffy.
With the mixer on low speed, slowly add the powdered sugar, followed by the heavy cream, vanilla, and dash of salt.
Continue to beat the frosting on medium-high speed for an additional 5 minutes.
Before frosting your cake, mix the frosting by hand with a wooden spoon to push out the air pockets.
Spread a dollop of frosting on a cake board or cake plate to act as "glue" to hold your first cake layer in place.
Place the first cake layer, top side up, in the center of the cake board or cake plate. Apply a rim of the brown butter frosting around the edge of the cake. This will act as a barrier to the marshmallow fluff and will help to support the cake layers as you stack.
Add about one cup of the marshmallow fluff and spread evenly within the frosting barrier.
Sprinkle Rice Krispie Treats over the marshmallow fluff and pat into place. If you're not serving your cake right away, omit the Rice Krispie Treats in between the layers and serve on top of the cake slices as you serve to your guests.
Place the second cake layer, top side up, on the fillings. Repeat steps with the frosting barrier, marshmallow fluff and Rice Krispie Treats.
Place the final cake layer, top side down, so the smoothest and straightest side is on top.
Apply a thin layer of frosting around the entire cake. This is the crumb coat. Freeze the crumb coated cake for about 10 to 15 minutes to set the frosting and crumbs in place.
After the crumb coat is set, continue to frost the cake with the remaining frosting.
Use the remaining marshmallow fluff and Rice Krispie Treats to decorate the top of the cake.
*This will make enough to fill the layers and pipe some on top of the cake