Preheat oven to 350° and grease 3 (9 inch) round cake pans. Line with parchment paper. Grease and flour parchment paper. Set aside.
Allow eggs to sit out at room temperature for 30 minutes before using.
In a small bowl, mix milk and vanilla together. Set aside.
In a medium bowl, beat butter until creamy. Add sugar. Mix until light and fluffy. Add flour and baking powder, into the butter mixture, alternating with the milk and vanilla mixture. Set aside.
Beat egg whites until stiff peaks form. Fold gently into cake batter. Pour into prepared pans. Spread with spatula to cover pans completely with batter.
Bake at 350° for approximately 30 minutes or until wooden tooth pick inserted in the center comes out clean. Place on cooling rack for 10 minutes then removed from pans, set cakes on cooling rack and discard parchment paper.
Buttercream Frosting:
Beat butter until light and creamy. Add milk and vanilla and mix well. Gently add powdered sugar and mix until well blended.
Strawberry Filling
Puree strawberries in a food processor and set aside.
In a small saucepan, whisk together sugar and cornstarch. Stir in water and mashed strawberries.
Bring to a boil over medium temperature, stirring constantly. Let the mixture boil and stir for 3 more minutes until thick and translucent. Set aside and let cool for 20 minutes.
To Arrange:
Place first cake layer on plate, frost with buttercream and then spread a layer of strawberry filling. Top with second cake layer and repeat. Finish with third cake layer and buttercream, filling on top. Be sure to frost sides of cake. Garnish with cut strawberries.