Gluten-free Reese’s Brownies

1 cup butter, melted (2 sticks)
3 cups sugar
1 Tbsp. pure vanilla extract
4 eggs
1 1/2 cups gluten-free all-purpose flour
1/2 tsp. xanthan gum (*if needed, see note)
1 cup cocoa powder
1 tsp. salt
1 1/2 cups chopped peanut butter cups (=about 14 Reese’s cups)
1/2 cup Reese’s peanut butter baking chips