Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal.
In a double boiler or microwave-safe bowl, combine the unsalted butter, semisweet chocolate chips, and chopped unsweetened chocolate. Heat gently until everything is melted and smooth, stirring occasionally.
In a large mixing bowl, whisk together the eggs, granulated sugar, vanilla extract, and espresso powder until well combined. Slowly add the chocolate mixture to the egg mixture, stirring constantly.
In another bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add this dry mixture to the chocolate-egg mixture, folding until just combined.
Chop up the Twix candy bars into bite-sized pieces and gently fold them into the brownie batter.
Pour the batter into your prepared baking pan, spreading it out evenly. Sprinkle the sliced Twix bar on top. Bake for 30-35 minutes, or until a toothpick comes out with a few moist crumbs.
Allow the brownies to cool in the pan for about 15 minutes, then use the parchment overhang to lift them out. Transfer to a wire rack to cool completely before cutting into squares.
