Preheat your oven to 350°F (175°C) and grease your 9x13 inch baking pan or line it with parchment paper for easy removal.
In a large mixing bowl, combine the unsalted butter and brown sugar. Using a whisk or electric mixer, cream them together until the mixture is light and fluffy.
Add the eggs, one at a time, mixing well after each addition. Then, stir in the vanilla extract until fully incorporated.
In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
Gently fold in the mixed chocolate chips, toffee bits, and Reese's Pieces until evenly distributed throughout the batter.
Pour the batter into the prepared baking pan, spreading it evenly with a spatula.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out clean.
Allow the bars to cool in the pan for about 10-15 minutes before transferring them to a wire rack to cool completely. Once cooled, slice into squares and serve!