Preheat the oven to 350 degrees F. Prepare a 9x13-inch pan by lining it with foil, using enough foil so that some drapes over the sides. Set the pan aside.
In a double boiler, melt the butter and chocolate chips together, stirring regularly. Once everything is melted, take the bowl off of the heat.
In a separate bowl, whisk the flour, cocoa powder, and salt together. A little at a time, add the dry ingredients to the chocolate mixture, alternating with the Guinness. Fold in the chocolate chips.
Pour the batter into the prepared pan and bake for 25-30 minutes, or until a knife comes out clean when inserted into the middle of the brownies. Let the brownies cool a bit while you make the fudge frosting.
In a medium sauce pan, melt the butter. Add the brown sugar and corn syrup and stir to combine. Add the sweetened condensed milk and mix. Do not boil the mixture. Only heat it up enough so that it will melt the white chocolate chips. Add the white chocolate and a pinch of salt. Remove the sauce pan from the heat and stir until they melt completely.
Top the brownies with chocolate chips.