Preheat oven to 350°F (175°C). Grease three 8-inch round cake pans, line bottoms with parchment, and grease parchment.
In a medium bowl, whisk together cake flour, baking powder, and salt.
In a large bowl, beat butter and sugar with an electric mixer on medium-high until pale and fluffy, about 3 minutes. Beat in eggs one at a time, then vanilla.
With mixer on low, add flour mixture in three parts, alternating with milk in two parts, beginning and ending with flour. Mix just until combined.
Divide batter evenly between two bowls (about 340g each). Stir cocoa powder into one bowl until no streaks remain.
Spread vanilla batter into two pans and chocolate batter into the third. Bake 22-25 minutes, until a toothpick comes out clean. Cool in pans 10 minutes, then turn out onto a wire rack to cool completely.
For buttercream, beat butter until creamy. Gradually add powdered sugar, strawberry powder, and salt. Beat in milk, 1 tablespoon at a time, until light and fluffy. Tint with food coloring if desired.
Place one vanilla layer on a plate. Spread with buttercream. Top with chocolate layer, more buttercream, and final vanilla layer. Apply a thin crumb coat, chill 20 minutes, then frost with remaining buttercream.