PREPARATION
Heat the oven to 325°C. Grease and flour an 8-inch square baking pan. 
In a saucepan, melt the butter and chocolate, over low heat, stirring constantly. Remove from heat and let it cool. 
Beat in the vanilla essence and sugar using a whisk or hand blender.
Add the eggs, one at a time, beating well after each addition.
Measure the flour into a small bowl, and mix in the baking powder and salt, blending well.
Using a wooden spatula, stir in the flour mixture into the chocolate mixture. Stir in the chopped nuts. 
Transfer this mixture into the prepared pan and spread it evenly.
Bake the brownies for about 25 minutes.