01 - Preheat oven to 350°F. Grease and line three 8-inch round cake pans with parchment paper.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl. Set aside.
03 - Beat butter and sugar until light and fluffy. Add eggs one at a time, then vanilla extract.
04 - Alternate adding flour mixture and milk to the batter, starting and ending with flour. Mix until just combined.
05 - Divide batter equally into three bowls. For chocolate layer, mix cocoa powder and 3 tablespoons milk until smooth, then fold into one portion. For strawberry layer, fold in strawberries or puree, extract, and coloring if desired. Leave third portion as vanilla layer.
06 - Pour each batter into prepared pans. Smooth tops and bake for 25-30 minutes until a toothpick inserted in center comes out clean.
07 - Let cakes cool in pans for 10 minutes, then transfer to wire rack to cool completely.
08 - Beat butter until creamy. Gradually add powdered sugar and salt, mixing well. Add vanilla and cream; beat until fluffy.
09 - Trim cakes for even layers if necessary. Place chocolate layer on serving plate, spread buttercream. Repeat with strawberry and vanilla layers.
10 - Cover entire cake with thin crumb coat of frosting. Chill 30 minutes, then frost with remaining buttercream. Decorate as desired with sprinkles, strawberries, or chocolate shavings.