Line a standard muffin tin with cupcake liners, then set it aside for now. Next, combine the softened unsalted butter and sugar in the bowl of a stand mixer. Cream them together until smooth, and scrape down the sides of the mixer as needed.
½ cup unsalted butter, ¾ cup sugar
Add the vanilla, eggs, milk, and oil into the mixer and mix them well.
¼ cup oil, 1 teaspoon vanilla, ½ cup milk, 2 eggs
Add the dry ingredients like flour, baking powder, and salt.
1 ½ teaspoons baking powder, ½ teaspoon salt, 1 ½ cups flour
Mix them thoroughly, scraping down the sides as necessary.
Using a spoon or ice cream scoop, divide the batter evenly between the cupcake liners, filling each until they're about 1/2 to 2/3 full. Place the muffin tin in the preheated oven and bake for about 14 minutes to allow the centers to set and the edges to brown slightly. Afterwards, take the tin out and allow the cupcakes to cool.
Let's start making the frosting! Add the softened butter and about half of the sugar to the stand mixer bowl. Then, mix them on low speed until combined.
1 cup unsalted butter, 4 cups powdered sugar
Add the remaining powdered sugar, vanilla, milk, and salt.
1 teaspoon vanilla, ½ teaspoon salt, 2 tablespoons milk
Mix on low speed to combine the ingredients, and then increase the speed to medium to whip the frosting until soft and creamy.
Next, divide the frosting into two separate bowls. Use food coloring to tint one blue and the other pink. Then, fit a piping bag with a large star tip. Add the blue frosting to one side of the piping bag, and the pink frosting on the other, so the bag contains both colors, allowing them to swirl when piped.
pink food coloring, blue food coloring
Check if the cupcakes are cooled completely. Once they are, we'll begin assembling the gender reveal cupcakes!
Use a teaspoon to remove the center of each cupcake.
Then, fill the hollowed centers of each cupcake with appropriate-colored sprinkles. Choose pink OR blue depending on the gender of the baby.
Next, grab the piping bag with the frosting and pipe a large dollop over the sprinkles, making sure they're completely concealed.
Lastly, top the frosting with additional sprinkles, and you're all set!
pink, blue, and white sprinkles
Notes
Don't overmix the batter. Basically, overmixing might make the cupcakes denser instead of tender, so just mix the batter until it's combined.
Fill cupcake liners evenly. Just fill each liner equally to about 2/3 full, so they all rise evenly without overflowing.
Let the cupcakes cool completely before filling. Adding the sprinkles too early might melt them or ruin the texture.
Add food coloring to the frosting gradually. Start with a small amount and build to your desired shade.
How to Store
Refrigerator: Keep any leftover frosted cupcakes inside airtight containers, then store them in the fridge for 2-3 days. Keep them in a single layer to avoid ruining the frosting.
Freezing: Unfrosted cupcakes can be frozen for up to 2 months. Only thaw and frost them once they're ready to be served.
Room temperature: They can be kept in an airtight container at room temperature for a day.