Pre-heat the oven to 350 degrees. Butter and flour a sheet pan (or use a nonstick cooking spray) and make a foil or parchment paper "sling" (if using aluminum foil, butter the foil as well).

In a large heat-proof bowl set over a pan of simmering water on low heat, melt the butter and both chocolates, stirring occasionally till completely smooth. Cool slightly.

In a separate bowl, lightly whisk the eggs. Add the espresso powder, vanilla and sugar, whisking till combined. Whisk into the melted butter chocolate mixture and cool to room temperature.

Chop seven of the Milky Way bars and thinly slice the remaining three bars.

Add the flour, baking powder and salt - fold in till just combined. Fold in the chopped Milky Way pieces and pour mixture into your prepared pan. Smooth top into an even layer with an offset spatula. Place the sliced Milky Way bars evenly on top.

Bake the brownies for 30 to 35 minutes, or until a toothpick inserted in center comes out with just a few crumbs attached (do not over bake). Let cool completely on a wire rack, then cover tightly and chill overnight (this is the hardest part but is the easiest way to make sure you get nice clean cuts!)

Use a thin knife to loosen the brownies from the sides of the pan. Using your sling, transfer the brownies in one piece to a cutting board. Use a sharp knife and cut into 48 bars. Store cut brownies chilled in an airtight container till about an hour before serving.