You can certainly use a chocolate ganache for your homemade cosmic brownies if you prefer it over making the cooked fudge frosting. I have made them both ways and they turn out good, I just prefer the fudge frosting in terms of taste and being more like the classic cosmic brownies. For a ganache, just heat 1/2 cup heavy cream in the microwave for 60 seconds until very hot. Pour the hot cream over 1 cup semisweet chocolate chips and 1/2 cup milk chocolate chips in a bowl with a pinch of salt and let this sit WITHOUT STIRRING for 3 minutes. Once the chocolate has had a chance to melt, whisk or stir until you have a thick, smooth ganache. Pour and spread over the brownies in place of the fudge frosting, then let cool until set.