1 1/2 cup all-purpose flour
1/2 cup cocoa powder
1 teaspoon baking soda
1/2 teaspoon salt
1 stick (8 tablespoons) unsalted butter, softened
3/4 cup Nutella
1/3 cup granulated sugar
1/3 cup dark brown sugar
1 large egg
2 tablespoons water
1 teaspoon vanilla extract

For the icing:
1 cup powdered sugar
1/2 teaspoon vanilla
2 tablespoons milk or water
Yellow food coloring
Red food coloring