You can never cross the ocean until you have the courage to lose sight of the shore. –Christopher Columbus
Adapted from Martha Stewart Cookies.
1/2 cup (1 stick) unsalted butter, room temperature, plus more for pan
2 1/4 cups all-purpose flour
2 1/4 teaspoons baking powder
1 teaspoon coarse salt
1 1/2 cups packed dark-brown sugar
3 large eggs
1 teaspoon pure vanilla extract
1 cup miniature marshmallows
1 cup semisweet chocolate chips
1/2 cup white chocolate chips
1 cup butterscotch chips
1/2 cup peanut butter chips
Preheat oven to 350 degrees. Lightly butter a 9-by-13-inch baking pan.
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, chocolates, butterscotch chips, and peanut butter chips.
Spread batter in prepared pan. Scatter remaining marshmallows, chocolates, butterscotch chips, and peanut butter chips on top. Bake until top is golden brown, about 35 minutes. Let cool on a wire rack. Cool completely before serving.
Bars can be stored in an airtight container at room temperature up to 1 week.