Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, or line them with parchment paper.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.

Beat in the eggs one at a time, ensuring each egg is fully incorporated before adding the next. Add the vanilla extract and mix well.

Gradually add the dry ingredients to the wet ingredients in batches, alternating with the milk. Start and finish with the dry ingredients. Mix until just combined.

Divide the batter evenly between two bowls. Add a few drops of food coloring to one bowl, mixing until you achieve the desired color (blue or pink). Leave the other bowl as is.

Start by adding a spoonful of the plain batter into the center of the first cake pan. Then, add a spoonful of the colored batter on top. Repeat this process, alternating colors, until both pans are filled. Gently tap the pans on the counter to level the batter.

Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center of the cakes comes out clean.

Remove the cakes from the oven and let them cool in the pans for 10 minutes. Then, transfer them to wire racks to cool completely.

Once the cakes are completely cooled, frost them with your choice of frosting. Decorate with sprinkles or edible glitter if desired.

When you're ready, cut into the cake and reveal the surprise inside!