Preheat the oven and prepare the pans.

Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans, ensuring they’re well coated.

Mix Dry Ingredients:

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

Cream, butter, and sugar:

In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3–5 minutes.

Add eggs and vanilla.

Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.

Combine the dry ingredients with milk:

Gradually add the dry ingredients to the butter mixture, alternating with milk, beginning and ending with the flour mixture. Mix until just combined.

Bake the cake layers:

Divide the batter evenly among the prepared cake pans. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

Prepare the buttercream frosting:

In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar, one cup at a time, beating well after each addition. Add the vanilla extract and 4 tablespoons of heavy cream. Beat on high for about 3–4 minutes until light and fluffy. If the frosting is too thick, add more cream, 1 tablespoon at a time.

Color the frosting:

Divide the frosting into separate bowls and tint each portion with your desired gel food coloring. Mix well to ensure an even color distribution.

Assemble the cake:

Level the cooled cake layers with a serrated knife, if needed. On a cake board or serving plate, place one cake layer. Spread a layer of frosting on top. Repeat with the second and third layers.

Shape the cake:

Using a serrated knife, trim the stacked cake to create a dome shape resembling the Barbie doll's skirt. Apply a thin layer of frosting (crumb coat) over the entire cake and refrigerate for 30 minutes to set.

Insert the Barbie doll:

Carefully insert the wrapped Barbie doll into the center of the cake, ensuring it’s secure and stands upright.

Decorate the cake:

Using a piping bag fitted with your preferred tip, pipe the colored frosting to create the Barbie doll's dress. Start from the bottom and work your way up, adding ruffles, swirls, or any design you like.

Add Final Decorations:

If using fondant, roll it out and drape it over the cake for a smooth finish. Add edible decorations such as pearls, glitter, or fondant flowers to enhance the design.

Serve and enjoy:

Once decorated, your Barbie birthday cake is ready to impress. Slice, serve, and enjoy the celebration!