Preheat the oven to 350°F (175°C) and grease an 8x8-inch baking pan with butter or cooking spray.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 2-3 minutes using a hand mixer.
Add the egg and vanilla extract to the butter mixture, mixing until fully combined. Stir in the cocoa powder, baking powder, and salt until evenly incorporated.
Gradually add the flour and milk in alternating batches, mixing gently until the batter is smooth and no lumps remain.
Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely in the pan before decorating.
In a small bowl, mix the powdered sugar with 1-2 tablespoons of water and orange food coloring to create a thick glaze. Spread the glaze evenly over the cooled cake.
While the glaze is still wet, press the black fondant balls onto the cake in a random pattern to resemble spider eggs or eyeballs. Allow the glaze to set before serving.