Preheat oven to 350 degrees. In microwaveable bowl, melt butter and chocolate until smooth.
Add sugar and mix till combined. Add eggs and vanilla and mix just till incorporated.
Add flour and stir until combined.
Stir in pecans (if using), half a bag of the chocolate chips and mini marshmallows.
Pour into greased 9x13 baking pan. Sprinkle with remaining chocolate chips.
Bake for 40-45 minutes. Cool completely before cutting into squares.