Preheat the oven to 180 degrees C (350F) and grease and line your baking tin.

Place the melted butter, sugar eggs and vanilla extract in a large bowl or the bowl of your stand mixer and mix until well combined.

Sift in the cocoa powder, flour and baking powder. Mix until well combined, about 1 minute on medium speed.

Add in the chocolate chunks and crush digestives, mix until evenly distributed.

Place the marshmallow fluff into a heatproof bowl and microwave for 10-20 seconds. Fold into the brownie batter, but not fully, you want a swirled, marble effect.

Tip mixture into brownie pan and smooth with a spatula/wooden spoon.

Place in the oven for around 30 minutes until slightly puffy and an inserted skewer comes out with a few crumbs on.

Leave to cool completely in the pan. Once cooled transfer to a cutting board and slice into 9 or 12 pieces.

Brownies will keep in an airtight container for 3 days.