Prep oven and pans. Position rack in the center oven. Preheat the oven to 350°F/177°C). Prepare three 8-inch (20 cm) round pans (or two 9-inch round pans) with non-stick spray, then line with parchment circle, and spray again.
Make strawberry puree. Puree 1 pound of strawberries either in a blender or food processor. Then in a saucepan, heat strawberry puree over low heat until reduced to about ½ cup. This can be done up to 2 days ahead of time. Refrigerate puree until ready to use (but make sure it’s at room temperature again before adding to cake batter).
Combine dry ingredients. Sift together cake flour, baking powder, and baking soda. Add salt and whisk together. Set aside.
Cream butter and sugar. In a stand mixer bowl with paddle attachment, (or you can use your hand mixer) cream butter and sugar together on medium speed until light and fluffy, about 5 minutes. Scrape down the bowl halfway through mixing. The mixture should be pale yellow.
Add extract and egg whites. With mixer on low speed, add in the vanilla. With the mixer running, slowly add in the egg whites mixing to combine. Scrape down the bowl as needed.
Alternate the dry ingredients and sourcream/milk. In a small bowl, stir together the milk and sour cream. Alternate adding the dry ingredients and milk/sourcream, beginning and ending with the dry ingredients. Mix on medium speed between additions until incorporated.
Bake cakes. Divide batter equally into the prepared pans. Bake for 22-26 minutes until cake is firm and springs back when gently touched in the center and a toothpick inserted into the center comes out clean. Cool cakes in the pan completely on a wire cooling rack before turning out of the pan. Once completely cooled, assemble and frost.