Start by preheating your oven to 350°F (175°C). Grease and flour a 9x13-inch rectangular cake pan, or use two smaller pans if you’d like to make two layers.

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

In a large bowl, beat together the softened butter and granulated sugar until light and fluffy. This should take about 3-4 minutes. The texture should be airy, and the color will be noticeably lighter.

Add the eggs one at a time, making sure each one is fully incorporated before adding the next. Mix in the vanilla extract.

Alternate adding the flour mixture and the milk into the butter mixture. Start with a third of the flour, then half the milk, mixing after each addition. Repeat until all ingredients are incorporated. The batter should be smooth and thick.

Step 6: Bake the cake

Pour the batter into the prepared pan, spreading it out evenly. Bake in the preheated oven for 30-35 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then remove from the pan and transfer to a wire rack to cool completely.

Once the cake is completely cool, cut it into a bottle shape. You can use a template for a more accurate shape or freehand it based on your desired proportions. The top should be rounded like a bottle’s nipple, the middle should be narrow, and the bottom should flare out wider.

In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar, a cup at a time, mixing thoroughly after each addition. Add the vanilla extract and milk (or heavy cream) and beat until smooth. If you want to make multiple colors, divide the frosting into separate bowls and tint with food coloring.

Frost the cake with your desired colors. Use light blue or pink for the bottle, and feel free to get creative with the frosting details for the nipple or other accents.