Preheat the oven to 350F degrees.
Lina 9x13x2 inch pan with parchment paper, or line with aluminum foil and lightly grease. Ensure there's an overhang over the sides - this makes it easier to lift the brownies out.
Add the butter, chopped chocolate and peanut butter to a large microwave-safe bowl. Microwave for 1 minute on medium power (not high), remove from the microwave and stir. Repeat the process until almost melted, then stir until smooth.
In a separate bowl beat together the eggs and sugar until glossy and lighter in color (about 2 minutes).
Whisk the egg mixture into the melted peanut butter & chocolate mixture a little at a time.
Carefully fold the cocoa powder, flour & salt into the batter using a rubber spatula or wooden spoon.
Fold in the peanut butter chips.
Pour the batter into the prepared pan and bake for 25-35 minutes - or until the top looks set and an inserted toothpick comes out clean or with a few damp crumbs. The middle of the pan shouldn't wobble if you nudge the pan.
Cool fully (about 3-4 hours) before cutting into squares with a thin, sharp knife.
*Store brownies in an airtight container at room temperature for up to 4 days. Brownies freeze well, then thaw overnight in the fridge.