Preheat oven to 300°F and prepare cupcake liners in a muffin tin.

Combine flour, sugar, cocoa powder, baking soda, and salt in a large bowl and set aside.

Whisk egg, milk, vegetable oil, and vanilla in a medium bowl until mixed.

Add wet ingredients to dry and mix until smooth. Incorporate water until batter is thin.

Fill cupcake liners halfway with batter and bake for 18-23 minutes; cool before frosting.

Beat softened butter until creamy. Mix in half of powdered sugar and vanilla.

Add remaining powdered sugar, heavy cream, and salt; beat until fluffy.

Separate some vanilla buttercream for white color, dye remaining green.

Mix cocoa powder into remaining buttercream for chocolate color.

Fill piping bags with chocolate and green buttercream. Pipe football and grass decorations on cupcakes.

Use white buttercream to pipe laces onto the footballs.

Serve cupcakes immediately or store in airtight container for up to 4 days.