In a medium saucepan, melt butter over medium-low heat.
1/2 cup butter
Add butterscotch chips, brown sugar, and stir until melted. Remove from heat. Allow to cool for 10 minutes.
1 package butterscotch chip, 1/2 cup brown sugar
In a small bowl, combine flour, baking powder, and salt. Stir. Set aside.
1 1/2 cup all-purpose flour, 1 tsp baking powder, 1/2 tsp salt
Add the beaten eggs to the cooled butter mixture and stir to combine.
2 eggs, beaten
Add the flour to the butter mixture, and stir until combined. Add in the vanilla. Set aside until completely cool.
2 tsp pure vanilla extract
Lightly grease a 9x13 baking pan, and lay parchment in, allowing some to hang over the sides.
When the butter and flour mixture has cooled, stir in chocolate chips, marshmallows, and (optional) chopped nuts. Spread in the baking pan.
1 package chocolate chips, 2 cups mini marshmallows, 1 cup chopped nuts
Bake in a preheated 350-degree oven for 28 minutes. Remove from the oven and let it cool completely before cutting into squares.
Notes
Make sure to melt the butter, brown sugar, and butterscotch chips on low heat. If it is too hot, the eggs will curdle when added to the mixture.Butterscotch chips do not melt easily and will require some stirring and patience on your part.