Place the cocoa powder in a large bowl, pour the boiling water over it and with a wooden sppon or rubber spatula stir until the cocoa comes together in a soft mass.Add the butter and beat with an electric mixer on low speed about 30 seconds until well combined. Add the confectioners sugar and vanilla and best on low until the sugar is incorporated, then beat on high about 2 to 3 minutes until the filling is light and fluffy. If needed add more boiling water 1teaspoon at a time if it is to thick.If its to thin add confectioners sugar 1Tablespoon at a time until filling is to your liking.