Heat the oven to 350 degrees and line a cupcake pan with four paper liners.
In a mixing bowl, beat the softened butter and granulated sugar until light and fluffy.
Add the egg and vanilla extract, then mix until fully combined.
Stir in the flour and baking powder, followed by the milk, until the batter is smooth.
Divide the batter evenly among the four liners and bake for 15 to 18 minutes, until a toothpick comes out clean.
Let the cupcakes cool completely on a wire rack.
In a clean bowl, beat the powdered sugar and remaining butter until creamy, then add green food coloring a drop at a time until the frosting reaches a grass-like shade.
Spread or pipe the green frosting on each cooled cupcake and press two candy pumpkins into the top of each one.