Preheat the oven at 170 degrees C.
Grease and line (parchment paper) the cake tin of 8x8 inch size.
In a bowl, sift together flour, cocoa and baking powder.
Melt together butter and chocolate in a double boiler (Heat quarter full of water in a deep pan, and place a heat proof bowl over the pan). Stir occasionally.
In a bowl, beat together egg and brown sugar till thick and creamy. Pour the chocolate mixture (just warm, not hot) in the egg mixture and mix well.
Gently mix the flour mixture and coffee to the chocolate egg mixture using a rubber spatula till well combined.
Fold the walnut in the mixture and pour it in the prepared tin.
Spread it evenly and bake it for 25-30mins or till done. The top of the brownie will have a thin shiny crust.
Cool it completely and then cut it in squares.
Transfer the brownies to a square glass baking dish.
Pour ¼ cup of strong espresso(with Vanilla essence) over the brownies and allow it to get soaked.
Combine 3 egg yolks and 1/4 cup sugar in a double boiler on a low heat.
Simmer for 10-12 mins, stirring continuously. Take it off the heat and whip the yolk till thick and creamy, this is called SABAYON ( in French) or ZABAGLIONE (in italian)
Mix the mascarpone cheese to the whipped yolk (Sabayon) and beat till combined well.
Beat the cream in another bowl till it forms a stiff peak.
Gently but thoroughly fold the whipped cream in the Sabayon Mascarpone mixture.
Spread this mascarpone cream mixture evenly over the soaked brownies. Refrigerate it for 4-5 hours or overnight.
Before serving, dust the Tiramisu with cocoa powder, decorate the way you want and serve chilled.
