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