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.
Notes
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.