Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.

Add buttermilk, oil, eggs, and vanilla extract to the dry ingredients. Beat until well combined.

Gradually add hot coffee to the batter, mixing until smooth. Batter will be thin.

Pour batter evenly into prepared pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Let cakes cool in pans for 10 minutes before inverting onto a wire rack to cool completely.

In a large bowl, beat butter until creamy. Gradually add powdered sugar, beating until light and fluffy.

Add milk and vanilla extract. Beat until smooth.

Level the top of one cake layer. Hollow out a circle in the center, about 2 inches in diameter, leaving about 1 inch of cake at the bottom.

Fill the hollowed-out center with pink or blue candies or frosting to reveal the gender.

Frost the top of the filled layer with frosting. Place the second cake layer on top.

Frost the entire cake with the remaining frosting.

Decorate with sprinkles or other decorations as desired.