Leprechaun Log with Easy Buttercream Icing

First, prepare the white cake mix according to the box directions, adding green food coloring to the batter. Next, grease a baking pan and sprinkle green sprinkles in the bottom. Pour the prepared batter into the pan. Add a few more drops of green food coloring to the batter in the pan and use a knife to swirl it around for a marbled effect. Bake the cake according to the cake box instructions (e.g., 350°F for 40 minutes). While the cake cools, prepare the buttercream icing. In a stand or hand mixer, beat the softened unsalted butter. Slowly add in the powdered sugar and mix until well combined on medium speed. Add in vanilla extract (or any desired flavor extract) and food coloring (if making colored icing like strawberry pink). Once the cake is completely cool, decorate it with the prepared icing and additional sprinkles.