Preheat your oven to 350 degrees F and prepare three 9-inch cake pans by lining them with parchment paper and spraying them with non-stick spray.
In a medium-sized mixing bowl whisk or sift together all-purpose flour, baking powder, kosher salt, and baking soda. Set aside.
In the bowl of a stand mixer add unsalted butter and granulated sugar. Cream the ingredients until light and fluffy using the paddle attachment, about 3-5 minutes.
Add in vanilla extract and almond extract and mix to combine.
Whisk together whole milk and plain yogurt. The mixture will be thick but that is ok.
With the mixer on low, alternately add the flour mixture and the milk/yogurt mixture.
When all the ingredients are combined mix just until thoroughly combined.
In another mixing bowl add egg whites and mix on high speed until light and fluffy.
Fold the fluffy egg whites and rainbow sprinkles into the cake batter just until combined.
Pour the batter into the prepared cake pans making sure to add an equal amount to each pan.
Bake for 20-25 minutes or until the centers spring back to the touch.
Let the cakes cool for 10 minutes in the pan, then remove them from the baking pans and let them cool on a cooling rack.
We will be using the double boiler technique for our frosting. First, boil some water in a small to a medium pot.
Place the egg whites and granulated sugar in a heatproof bowl that fits over the pot without touching the water.
Using a whisk, mix continuously and add the vanilla extract.
Keep whisking until the sugar is dissolved into the egg whites.
Once heated to 150 degrees F and the meringue is formed, let it cool to room temperature for 15-20 minutes.
Once cooled, turn the mixer on low, and begin adding small pieces of salted butter.
When the mixture looks thick, put it on high for 30 seconds until well mixed.
Pipe some buttercream onto your first layer of cake.
Repeat with your second layer, then place the third layer on top.
Cover your cake with the rest of your buttercream, add rainbow sprinkles, and enjoy!