In a medium bowl or measuring cup, combine and stir the egg whites, whole egg and the vanilla. Set aside.

In a bowl of an electric mixer, fitted with a paddle attachment, combine the dry ingredients together on low speed for 30 seconds.

Add the butter one piece at a time, about every 10 seconds. Once all the butter is added, pour in the buttermilk and mix on low for about 1 minute, until the ingredients are incorporated.

Scrape down the sides of bowl and begin to add the egg mixture in 3 separate batches, mixing on miedum-low until light and fluffy, about two minutes. Fold once or twice to ensure the batter at the bottom of the bowl is incorporated and mix for another 20 to 30 seconds.

Let the cake layers cool on racks for 10 minutes before inverting onto greased wire racks. Gently turn the cakes back up so the tops are up and cool completely.

Once the cakes are cooled completely, level the tops if needed.

Wrap each cake layer with plastic wrap and chill in freezer for at least an hour before using. The cake layers can be stored for up to a few days wrapped once in plastic wrap and frozen. To store longer than a few days, wrap twice in plastic wrap, then in foil, and seal in a zip-lock bag.

In a medium size saucepan, combine the strawberries, rhubarb, sugar and vanilla bean paste over medium heat. Bring to a boil.

Once the mixture is boiling, reduce the heat to low and let simmer, stirring occasionally, until the fruit is tender.

Let cool completely before using in the cake. Can be made ahead of time and stored in an airtight container for up to a week.

Preheat your oven to 375 degrees F. Line a baking sheet with parchment paper. Set aside.

In a medium size bowl, combine the flour, sugar, salt and cinnamon. Whisk to blend.

Add the cubes of butter and rub in with your fingertips. The mixture will stick together like clumps. Mix in the oats and nuts. 

Bake for 8 to 10 minutes, stirring the crumble halfway through. Let cool completely before using in the cake. 

In the bowl of a stand mixer fitted with the paddle attachment cream the 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 bean paste, vanilla extract 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.

Place the first cake layer, top side up (crumb side up), on a cake board or cake plate. Using an offset icing spatula, spread a thin layer of frosting over the cake layer. This creates a barrier between the cake and fruit filling so the cake doesn't become soggy. 

Pipe a rim of frosting around the edge of the cake layer. This will help support the cake layers and prevent the fruit filling from spilling out. (You may even need to pipe a second time over the first rim you made - refer to the video above).

Spread about 1/2 cup of the fruit filling on the cake layer. 

Sprinkle about half of the crumble over the fruit filling.

Gently place the second cake layer on top of the filling and crumble and repeat steps 3 to 4.

Place the final cake layer top side down on the second layer of filling. 

Freeze the cake for about 10 minutes to help set the frosting and filling, making it a bit more stable for when you frost. If the cake still feels wobbly, add a dowel to the center of the cake (or you can even use drinking straws for this).

After the filling is set, use a small amount of frosting to apply a crumb coat around the entire cake. Freeze the cake again for another 10 minutes to set the crumb coat.

After the crumbs are locked in, continue to frost and decorate the cake.