Given the task definition and input, reply with output. In this task, you will be presented with the directions of a recipe separated by "," and have to fill in the "___" which is a step that is missing from the recipe.

In a large mixing bowl, combine the flour and sugar.,In heavy saucepan, combine butter, shortening, coffee, and cocoa.,Stir and heat to boiling.,Pour boiling mixture over the flour and sugar in the bowl.,Add the buttermilk, eggs, baking soda, and vanilla.,Mix well using a wooden spoon, or high speed on an electric mixer.,______,Bake at 400F for 20 minutes, or until brownies test done in the center.,While brownies bake, prepare frosting.,In a saucepan, combine the butter, cocoa and milk.,Heat to boiling, stirring.,Mix in the powdered sugar and vanilla until frosting is smooth.,Pour warm frosting over brownies as soon as you take them out of the oven.,Cool.,Cut into 48 bars.,TIP: If you don't have buttermilk on hand, substitute 1/2 cup milk with 2 teaspoons vinegar or lemon juice.,Or you can use powdered buttermilk.
Pour into a well buttered 17x11 jelly roll pan.