Preheat oven to 350 degrees. Grease an 8x8x2 inch baking pan and set aside.
In a medium bowl, mix together flour, granulated sugar, baking powder and salt.
Stir in milk, melted butter and vanilla. Mix well. Add the melted chocolate and chopped nuts. Stir well.
Spread the batter out evenly in the prepared pan.
Mix brown sugar and cocoa powder together in a small bowl. Sprinkle evenly over the top of the brownie batter.
Slowly pour boiling water over the top.
Bake for 35 or so minutes, until top is crackly, but bottom is still gooey. Serve warm with vanilla ice cream or frozen yogurt.