Preheat oven to 350°F. Lightly grease a 9-inch spring foam pan, and line the bottom with parchment paper(cut out to fit the bottom). Place a heatproof bowl over a pot of simmering water/double boiler(make sure the bowl does not touch the water). Melt the butter and semi-sweet chocolate until smooth. Add the sugar and vanilla and stir until the sugar is slightly melted, remove from heat. In a small bowl, whisk together the flour, cocoa powder, and salt. Add flour mixture and two eggs to the chocolate mixture and mix until combined. Pour and evenly spread around prepared pan. Bake for 15-18 minutes, or until a tooth pick inserted in the center comes out clean. Cool completely.
In a double boiler, melt the 3tbs of butter, 8oz of chocolate, and stir until smooth, add bourbon and stir till just combined. Turn off heat. When the chocolate is not hot, but warm, add the 1 egg yolk and gently stir into the chocolate, stop when the yolk is incorporated. Chill a mixing bowl and whisks, when chilled, beat on high speed, the 3/4 cup of cream till soft peaks form. Set aside. With an electric mixer with whisks attached(not chilled), beat the 3 egg whites on high speed until soft peaks form, add the 2tbs of sugar, and return to high speed until stiff peaks have formed. In three batches, fold the whipped cream into the chocolate(it should be just warm to touch), making sure each addition is fully combined. Now gently fold in the egg whites, in three batches, until fully combined. When the brownies are fully cooled in the pan, gently pour the mousse on top and spread around evenly. Cover tightly with plastic wrap and refrigerate at least 6 hours or overnight(best to leave in overnight). When the mousse has set, gently remove the ring from around the spring foam pan. Pour the ganache over top and let sit in the fridge to firm, 30 minutes to 1 hour. Cut and serve.
Chop the chocolate into small pieces and place in a large bowl. Heat cream in a saucepan and bring to a simmer. Remove from heat and instantly pour over the chopped chocolate. Sit for 1 minute and then stir till chocolate is melted and is smooth. Let it sit for 10 minutes then pour over the set mousse.