Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
Mix dry ingredients: In a bowl, sift together flour, baking powder, and salt. Set aside.
Cream butter and sugar: In a large mixing bowl, beat butter and sugar until light and fluffy.
Add eggs and vanilla: Beat in eggs one at a time, then add vanilla extract.
Alternate adding flour and milk: Gradually add dry ingredients and milk alternately, beginning and ending with flour mixture. Mix until just combined.
Divide batter: Split batter evenly into two bowls. Color one bowl with pink food coloring and the other with blue.
Bake layers: Pour batter into prepared pans and bake for 30-35 minutes or until a toothpick inserted comes out clean. Let cakes cool completely.
Assemble cake: Stack cake layers with buttercream frosting in between, alternating colors to keep the surprise inside. Frost the entire cake with neutral or white buttercream.
Decorate: Add sprinkles or edible decorations on top for a festive look.
Reveal: Cut the cake in front of guests to reveal the gender inside!