Chop the butter and chocolate up into small pieces and put in a bowl. Microwave on high for 1-2 minutes until melted. Allow to cool.
125 g unsalted butter, 125 g good quality dark chocolate
Whisk the light brown sugar and eggs together for 3-5 minutes using an electric mixer, until it looks thick and creamy.
185 g light brown sugar, 2 eggs
Pour the cooled chocolate and butter mixture over the egg and sugar mixture and fold it in gently until completely mixed.
Sieve in the flour and again fold in gently until completely incorporated, but do not overmix.
80 g plain flour (all-purpose flour)
Grease and line your slow cooker bowl with grease proof paper.
Pour the brownie batter in.
Mix the butter, peanut butter and icing sugar together in a small bowl, until smooth.
40 g unsalted butter, 100 g smooth peanut butter, 40 g icing sugar
Use a spoon to dollop it on top of the brownie batter.
Swirl it together with the brownie batter with a knife.
Cook on high for 2-3 hours until just set in the centre. Allow to cool completely before using the baking paper to remove from the slow cooker.
Notes
Note: Timings depend on your slow cooker. Check after an hour if you have a slow cooker that cooks fast (e.g. a sear and stew). If you have a slow one like me it could take 2-3 hours.