Grease a 9x13 baking dish and pour in the cake batter. Bake cookies at 350 for 30 minutes, or until golden brown and set in the center. Center will still be soft. Allow to cool completely before icing.
To make the icing
In a large bowl, cream the butter and about 1 cup of confectioner's sugar with an electric mixer until light and fluffy. Add the butter flavoring, half of the milk, and remaining sugar and beat again until smooth and creamy, scraping down the sides and adding a little extra milk if needed.