Preheat oven to 350°F (175°C) and line a cupcake tin with green liners.

In a bowl, whisk together flour, sugar, baking powder, and salt.

In another bowl, cream the butter until fluffy; gradually add the sugar.

Beat in eggs one at a time, then mix in milk and vanilla until smooth.

Gradually combine dry ingredients with wet mixture until just blended. Add food coloring to achieve desired shade.

Fill liners two-thirds full and bake for 18-20 minutes or until a toothpick comes out clean. Cool completely before frosting.

In a bowl, beat together powdered sugar and unsalted butter until smooth and spreadable.