Spray an 8x8 or 9x9 baking pan with cooking spray and line with parchment paper. Let paper extend over two sides for easy removal of brownies.
Melt bittersweet chocolate and butter together in a microwave safe container. Heat on high for 30 second, stir. Repeat. Let sit for a few minutes and stir again. Chocolate should be smooth and completely melted. If not, heat on 50% power for in 15 second intervals until melted.
Beat eggs and sugar together in a medium bowl with a fork.
Mix in salt and vanilla extract. Stir in melted chocolate mixture.
Stir in the flour just until combined.
Stir in peanut butter chocolate chips, marshmallows, and semi sweet chocolate chips.