Grease and flour (2) 9” round cake pans. Line bottoms of pans with parchment paper; set aside. Preheat oven to 350°F.

Using a medium bowl, sift together the flour, baking powder, and salt. Stir until well combined; set aside.

Using a countertop mixer fitted with the paddle attachment, cream together the butter and sugar until light and fluffy (~2-3 minutes on medium speed). Add the egg and egg whites; mix on low speed until well combined.

Add half of the flour mixture to the bowl; mix on low speed until well combined.

Add the milk, sour cream, vanilla extract, lemon juice and lemon zest; mix on low speed until well combined. Add the remaining flour mixture; mix on low speed until well combined. (Tip: Don’t forget to scrape down the sides of the bowl to ensure all ingredients are fully mixed.)

Divide the batter evenly into the 2 prepared cake pans.

Bake at 350°F for 32-34 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Let cakes cool in pan for 10 minutes before turning out onto a cooling rack until completely cool.

Using an electric mixer, beat the cream cheese, powdered sugar, lemon juice and lemon zest until smooth; set aside.

Whip the cream until stiff peaks form. Fold the whipped cream into the cream cheese mixture.

Using a medium bowl, add all ingredients and stir until well combined.

Line a baking pan with foil and spread crumb topping on top. Bake at 350°F for 8-10 minutes. Let crumb topping cool completely and then crumble into small pieces.

Place one layer of cake on a large plate.

Remove ¾ cup of filling and set aside. Spread the remaining frosting evenly on the bottom layer of cake.

Add second layer of cake. Use remaining frosting to frost sides of cake. Gently press crumbled topping on top.

Refrigerate cake for at least 2 hours prior to serving.