Preheat the oven to 350F (180C or 170C fan forced).
Line an 8x8 inch baking pan with parchment paper or aluminum foil, leaving an overhang around the edges.
Add the butter and chopped chocolate to a large bowl. Microwave on medium power (not high power, which is often the default) for 45 second intervals, stirring between each interval, until smooth. Alternatively, melt together in a double boiler.
Allow the mixture to cool slightly. Then whisk in the sugar and eggs.
Whisk in the flour, cocoa and salt until the batter is smooth. If the cocoa powder looks lumpy, I recommend sifting it first.
Pour about 1/2 of the batter into the prepared pan.
Place 9 unwrapped peanut butter cups on top in 3 rows of 3 peanut butter cups and press down slightly.
Drop spoonfulls of batter over the peanut butter cup layer.
Then spread the batter smooth so that the peanut butter cups are covered.
Bake in the preheated oven for 27-32 minutes, or until the top looks set and an inserted toothpick comes out clean or with melted peanut butter.
Cool the brownies fully (about 3-4 hours) before slicing. Then lift out of the pan using the overhang from the parchment/aluminum foil. Place on a cutting board and slice with a sharp knife (not a table knife).