Preheat oven to 350°F / 175°C. Line a 9x13 baking pan with parchment paper. Set aside.

Stir flour, baking powder, and salt to combine. Set aside.

Combine milk, egg whites, and vanilla* and stir to combine. Set aside.

In a large mixing bowl, mix butter and sugar with a handheld or stand mixer fitted with a whisk or paddle attachment on medium speed until creamy. Alternately add dry and wet ingredients, beginning and ending with dry ingredients. Mix on low speed just until combined. 

Separate batter in 2 bowls and color one with pink and one with red food coloring. Alternately transfer batter to the prepared baking pan and draw a few swirls with a knife to get a beautiful pattern. Bake for 25-27 minutes or until a toothpick in the center comes out clean. Let cool to room temperature (about 30 minutes).

Use a 1.6-inch (4cm) heart-shaped cookie cutter and cut out cake hearts. You need about 24-26 hearts for the cake. Set aside. Save the rest for other baking projects like cake pops, or just eat them.

Preheat oven to 350°F / 175°C. Line two 8-inch baking pans with parchment paper by cutting 8-inch circles for the bottoms and about 2-3 inch wide strips for the sides. Use very less baking spray underneath the paper that it sticks to the pan for easier filling.

Stir flour, baking powder, and salt to combine. Set aside.

Combine milk, egg whites, and vanilla* and stir to combine. Set aside.

In a large mixing bowl, mix butter and sugar with a handheld or stand mixer fitted with a whisk or paddle attachment on medium speed until creamy. Alternately add dry and wet ingredients, beginning and ending with dry ingredients. Mix on low speed just until combined. 

Spoon a few spoons of batter in the two pans equally until the bottom is thinly covered with batter. Place about 12-13 white cake hearts in a circle in each pan upside down. Spoon remaining batter on top of the hearts. Bake for about 27-30 minutes or until a toothpick in the center comes out clean. Let cool completely.

Blend strawberries with a handheld emersion blender until smooth. Place a small saucepan over medium-high heat and set a fine mesh strainer on top. Pour strawberries through the strainer into the saucepan. Press fruit and liquid through the strainer and discard solids. Bring to a boil and cook until it is reduced by half, for about 10 minutes. Let cool to room temperature. 

Mix butter with a whisk attachment on medium speed until creamy for about 2 minutes. Alternately add 1 cup sugar and 1 tbsp strawberry puree at the time and mix on low speed until combined. Add salt and vanilla* and mix until smooth and fully incorporated. If the frosting is too thick to spread, add 1 tbsp milk at the time until you reach the desired consistency.

Place bottom cake layer with a cake board underneath on a turner. Pipe or spread buttercream on top. Even with a spatula. Place the second cake layer on top. Pipe or spread remaining buttercream on top and the sides until the cake is fully covered. Level the top and the sides with the spatula until buttercream is smooth and even. Smooth the edges by leveling the top and push frosting very slightly and carefully to the edges. Then level the edges on the sides again and repeat until edges are smoothed.

Chill in the refrigerator for at least 3 hours. Decorate as desired before serving. Store leftovers in an airtight container in the fridge up to 3 days.