Preheat the oven to 350°F. Spray a 12x18x1-inch half sheet pan with nonstick spray. Line with foil or parchment that hangs over the sides for ease in getting them out (spray the foil/parchment with nonstick spray).
In a heatproof, medium-bowl set over a saucepan of simmering water, heat the butter, and chocolates until melted and smooth; cool slightly.
In a large bowl, whisk together the sugar, eggs, vanilla and espresso powder. Add the cooled chocolate mixture to the egg mixture and whisk to combine.
In a medium bowl, sift together the flour, baking powder, and salt, then add it to the batter. Stir the chopped mini Milky Way bars into the chocolate mixture. Then pour the batter into the prepared baking pan and smooth the top with a rubber spatula. Place slices of the full-size Milky Way Bar onto the top of the brownie batter.
Bake for 25 to 35 minutes, or until a toothpick inserted into the center comes out clean with a few crumbs attached; do not overbake! Let cool completely, then cover tightly and chill overnight (or for several hours). Chilled brownies are easier to cut.
*These taste better the second day.
*If it's a warm day inside your house, you might consider chilling the Milky Way bars in the refrigerator before you try to cut them.