Preheat the oven to 350°F. Butter and flour a 12×18x1-inch (half-sheet) pan. Line with foil or parchment that hangs over the sides for ease in getting them out (butter the foil/parchment).
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 eggs, espresso powder, vanilla and sugar. Stir the egg mixture into the slightly cooled chocolate mixture. Cool to room temperature.
In a medium bowl, sift together the flour, baking powder, and salt; add to the batter. Stir 12 ounces chopped Twix bars into 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 Twix 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 before cutting. My best tip is to cover the brownies tightly and chill overnight. 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 Twix bars before you try to cut them.