Preheat oven to 350ºF. Spray pan with vegetable pan spray.
In medium bowl, combine flour, baking powder and salt.
In large bowl, cream butter and sugar with electric mixer until light and fluffy. Add egg and vanilla; mix well. Add flour mixture alternately with milk, mixing only until combined. Add green icing color 1 or 2 drops at a time until you reach your desired shade. Pour into prepared pan.
Bake 25-30 minutes or until toothpick inserted into the cake comes out clean. Cool in pan 10 minutes. Turn cake out onto cooling rack. Cool completely.
Fill holes in cake with chocolate candy pieces to resemble ornaments and pipe buttercream using a round tip to make garland.