Preheat oven to 350 degrees F.
Grease a 9 by 13-inch cake pan (aluminum is fine) with butter. Beat the 1 1/2 sticks butter and the sugar together in a large bowl until blended. Beat in the eggs 1 at a time, then stir in water and vanilla. Sprinkle the salt and baking powder over the mixture, then mix in. Do the same with the cocoa. Finally, stir in the flour until just blended.
Put the candy bars in a food processor or blender and pulse on low speed until all the bars have been reduced to a coarse crumble. Fold the crumble into the batter thoroughly. Scrape the batter into the prepared pan. Bake for about 30 minutes, until the center is set, the edges look a bit crusty, and the top of the brownies start to crack a little. Cool completely before cutting into squares.