Preheat the oven to 160°C/320°F (140°C fan). Line a 20cm square baking tin with baking paper, leaving an overhang on each side.
Add the butter, sugar and cocoa powder to the Thermomix bowl and melt for 3 min/50°C/speed 2.5.
Set the Thermomix to 1 min/speed 3 and drop the eggs in one at a time through the lid, keeping the machine running for the full minute. Scrape down the sides of the bowl with a spatula.
Tip in the flour and mix for 3 sec/speed 5. Scrape down the sides of the bowl and mix for a further 3 sec/speed 5 until well combined. Stir in any loose bits of flour with a spatula.
Pour the batter into the prepared tin. Melt the peanut butter in the microwave for approximately 30 seconds until runny, then drizzle it over the top of the chocolate batter. Use the tip of a knife or skewer to swirl it through. (See the photo in the post for guidance.)
Bake for 30-35 minutes until the centre is just set. Leave to cool completely in the tin before cutting into squares or bars.
Notes
Peanut butter: Both commercial (like Skippy or Bega) and natural peanut butter work here. Commercial peanut butter needs microwaving until runny before drizzling. Natural peanut butter often separates, with oil rising to the top - give it a good stir first, then, if it's still stiff, microwave for 30-40 seconds. A loose paste is fine to dollop and swirl through with a skewer.Storage: Keep the brownies in an airtight container somewhere cool for up to 3 days. To freeze, wrap each square individually in cling film, then store in an airtight container in the freezer for up to 3 months. Thaw at room temperature before serving.