STRAWBERRY FILLING: Using a fork, mix the cornstarch and water together in a small bowl until the cornstarch has dissolved—mixture is thick. Warm the strawberries and sugar together in a small saucepan over medium heat. Stir the mixture as it cooks, and break up some of the strawberries as you stir. Bring it to a simmer. Once simmering, stir in the cornstarch mixture. Allow to simmer for 5 minutes, stirring constantly. After 5 minutes, remove from heat, stir in the lemon zest, and cool completely. You will have about 1¼ c (about 340g) of strawberry filling. The filling thickens as it cools. (It must completely cool, so transferring to a bowl out of the saucepan helps speed that up, as does refrigerating it. You can complete a few other steps in the recipe as the mixture cools.) DO AHEAD: You can make the strawberry filling and store it covered in the refrigerator for up to 2–3 days before using.

CAKE: Preheat the oven to 350°. Grease three 6"×2" cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans.

In a large bowl, whisk the flour, baking powder, baking soda, and salt together. Set aside. In a medium bowl, whisk the melted butter and sugar together. Mixture will be gritty. Whisk in the egg whites, sour cream, milk, and vanilla bean paste/extract until combined.

Pour the wet ingredients into the dry ingredients and whisk until the batter is completely combined. Batter should be creamy and mostly smooth; a few small lumps are OK.

Pour batter evenly into cake pans. Bake for around 18-21 minutes or until the cakes are baked through. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, it’s done. Allow cakes to cool completely in the pans set on a wire rack. The cakes must be completely cool before frosting and assembling. DO AHEAD: Cake layers can be baked, cooled, and covered tightly at room temperature overnight.

STRAWBERRY WHIPPED CREAM: Using a hand mixer or a stand mixer fitted with a whisk attachment, whip the heavy cream, sugar, and vanilla bean paste/extract on medium-high speed until medium peaks form, about 3–4 minutes. Medium peaks are between soft/loose peaks and stiff peaks, and are the perfect consistency for topping and piping on desserts. If you accidentally over-whip the cream, and it looks curdled and heavy, pour in a little bit more cold heavy cream, and fold it in gently by hand with a spatula until it smooths out. Fold in the ½ c of strawberry filling. DO AHEAD: Cover tightly and chill in the refrigerator for up to 24 hours.

ASSEMBLY: Using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Discard (or crumble over ice cream!). Place 1 cake layer on your cake stand, cake turntable, or serving plate. Evenly cover the top with about ½ of the remaining strawberry filling. Top with 2nd cake layer and evenly cover the top with remaining strawberry filling. Top with the third cake layer. Spread the remaining strawberry whipped cream all over the top and sides. A bench scraper and small offset spatula are handy for decorating. Decorate with sliced strawberries and/or fresh mint. DO AHEAD: Frosted cake or unfrosted cake layers can be frozen up to 2-3 months. Thaw overnight in the refrigerator and bring to room temperature before decorating/serving.

Refrigerate for at least 30-45 minutes before slicing. This helps the cake hold its shape when cutting.

Cover leftover cake and store in the refrigerator for 5 days.