Step 1: Prepare the Cake Batter

1. Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans, or line them with parchment paper.
2. In a medium bowl, whisk together the all-purpose flour, baking powder, and salt.
3. In a separate large bowl, cream together the butter and sugar until light and fluffy, about 3-5 minutes. This is key to getting a light, airy cake!
4. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
5. Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Begin and end with the dry ingredients. Mix until smooth.
6. Add a few drops of your chosen food coloring (pink or blue) and mix gently to incorporate. This color will be the hidden surprise!

Step2: Bake the Cake

1. Divide the batter evenly between the two prepared cake pans.
2. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

Step 3: Prepare the Buttercream Frosting

1. In a large bowl, beat the butter until creamy. Gradually add the powdered sugar, one cup at a time, beating well between each addition.
2. Add the vanilla extract and milk to achieve a smooth, spreadable consistency. If you want to color the frosting, add a few drops of food coloring to match the theme of your event.

Step 4: Assemble the Cake

1. Once the cakes are completely cool, slice a small amount off the top of each cake to make them flat.
2. Place the first cake layer on your serving plate. Carefully cut a well into the center of the cake, about 1-inch deep, to hold the surprise filling.
3. Fill the well with your chosen pink or blue candy (M&Ms, sprinkles, or nonpareils).
4. Place the second cake layer on top, pressing down gently to seal in the surprise.

Step 5: Frost the Cake

1. Cover the entire cake with a thin layer of buttercream frosting to create a crumb coat. Chill the cake in the fridge for 20-30 minutes.
2. After the crumb coat has set, apply the final layer of frosting, smoothing it out for a neat finish.
3. Decorate the cake with additional frosting swirls, fondant decorations, or sprinkles to match your theme.