Preheat oven to 350 degrees. Line a 9x13 baking pan with foil, leaving an overhang on two opposite sides (you'll use the foil overhang as handles to lift the bars out of the pan). Spray the foil with cooking spray.

Whisk together flour, baking powder, and salt in a medium bowl. In the bowl of an electric mixer fitted with the paddle attachment, mix butter and brown sugar until fluffy, about 2 minutes. Add eggs and vanilla; mix until well combined. Mix in flour mixture until combined. Fold in half of each of the marshmallows, chocolate and toffee chips.

Spread batter evenly in prepared pan. Scatter remaining marshmallows, chocolates and toffee on top. Bake until top is golden brown and a cake tester inserted into the center comes out clean, about 25 minutes. Let cool on a wire rack. Lift out of pan, and transfer to a baking sheet. Refrigerate until set, at least 30 minutes.

Remove parchment, and cut into 20 squares. Bars can be stored in an airtight container at room temperature up to 1 week.