Preheat oven to 350°F (175°C). Line a muffin tin with cupcake liners.
In a large bowl, sift together flour, cocoa powder, baking powder, baking soda, and salt.
In another bowl, whisk eggs, light brown sugar, granulated sugar, vegetable oil, melted butter, buttermilk, whole milk, and vanilla extract until combined.
Gradually add dry ingredients to the wet ingredients, stirring gently until just combined. Avoid overmixing.
Fill each cupcake liner about two-thirds full with batter.
Bake for 15-18 minutes, or until a toothpick inserted in the center comes out clean.
Remove from oven and let cupcakes cool in the pan for 5 minutes. Transfer to a cooling rack and cool completely before frosting.
For frosting, beat softened butter until creamy. Gradually add confectioners’ sugar, alternating with milk, and beat until smooth. Stir in vanilla extract.
Once cupcakes are fully cooled, frost as desired using a piping bag or spatula. Decorate with Halloween sprinkles or themed decorations.