Combine the milk and cereal in a medium bowl. Let sit for about 20 minutes at room temperature.

Strain the milk through a fine mesh sieve or strainer, collecting the milk in a small bowl. Collect 1 cup.

Whisk the brown sugar and salt into the milk until fully dissolved. 

Preheat your oven to 350 degrees F. Spray three 8-inch or four 6-inch round pans with nonstick spray and line the bottoms with parchment paper. Spray again and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, combine the sugar, flour, baking powder and salt. Stir to combine. 

With the mixer on medium low, gradually add the butter and milk. Stir until the mixture is smooth, about 1 minute. Scrape down the sides of the bowl.

Add the egg whites and clear vanilla extract in three batches, scraping down the sides of the bowl after each addition.

Evenly divide the batter among the three pans. Bake for about 25 minutes. Let cool in the pans for about 10 minutes before inverting onto cooling racks to cool completely. Can wrap each cake layer individually with plastic wrap and freeze for up to a week.

Preheat your oven to 275 degrees F. 

Pour the Fruit Pebbles into a medium bowl and crush them a little with your hands. Add the milk powder, sugar, salt, and toss to mix. Add the melted butter and toss to coat. As you toss, the butter will act as glue, binding the dry ingredients to the cereal and creating small clusters.

Spread the clusters on a parchment lined sheet pan and bake for 20 minutes. Cool completely before using or store in an air-tight container in the freezer for up to a month.

In the bowl of an electric mixer fitted with the paddle attachment, add the butter and beat on medium speed for 2 minutes, until nice and fluffy. 

With the mixer on low speed, gradually add the powdered sugar. Mix until combined.

Add the cream, salt and clear vanilla. Turn the mixer to medium-high and beat for 5 minutes.

Transfer 2 cups of the frosting to a medium bowl. 

With the remaining frosting, beat by hand with a wooden spoon to push out any air pockets before frosting the cake.

In a blender or food processor, pulse the Fruity Pebbles cereal into fine crumbs.

Combine the Fruity Pebbles cereal and 2 cups of frosting and stir until combined. The filling will now be a little thicker than the frosting. It will spread more like a cookie dough filling. I suggest making the filling when you're ready to use it.

Place your cake board on a turntable. Spread a small dollop of frosting on the cake board and place the first cake layer, top side up, on the center of the cake board.

Carefully spread half of the Fruity Pebbles filling on the cake layer, followed by 1/2 cup to 1 cup of the Fruity Pebbles Crunch.

Place the second cake layer, top side up, on the fillings and repeat step 2.

Place the final cake layer, top side down, on the second layer of fillings. With the vanilla frosting, lightly frost the cake to create a crumb coat. (This layer of frosting is meant to be thin and does not need to be perfect. The point of the crumb coat is simply to lock in the crumbs.) Freeze the cake for 10 minutes to set the crumb coat. 

Continue frosting the cake with the remaining frosting and extra Fruity Pebbles Crunch. Best served day of assembly. Can also be frozen in an airtight container for up to a week.