Line a 10" x "12 or 13" x 9" baking pan with parchment paper.2.
Melt the dark chocolate with butter and Guinness in a double boiler or a metal mixing bowl set over a simmering pot of water, stirring well but not over-mixing.3.
Whisk the sugar and eggs until white and fluffy. Pour melted chocolate into the whisked eggs and fold together.4.
Sift the flour and cocoa powder into the mixture and fold in. Pour the mixture into the baking pan and sprinkle with sea salt and bake at 320 degrees F for 18 to 20 minutes. For best results, place a small pan of water at the base of the oven to keep brownies moist.5.
After 18 to 20 minutes, check to make sure the brownies are cooked — they should be firm to the touch but not hard. If the brownies are still soft in the middle, continue to cook until firm, checking every 2 minutes. Once cooked, remove pan from the oven and let brownies cool while you prepare the icing.6.
For the icing, melt the white chocolate over a double boiler or in a metal mixing bowl set over a pot of simmering water. Add a few drops of green food coloring and 2 teaspoons of peppermint syrup. If the chocolate starts to crack or separate, add a little vegetable oil to bring it back together.7.
Once the chocolate peppermint icing is melted and the brownies have cooled completely, spread icing over the top of the brownies using a rubber spatula. Sprinkle with gold sanding sugar (optional). Slice and serve.