Dice 125 g butter into small chunks and put it in a microwaveable bowl with 125 g milk or dark chocolate (or a mixture). Microwave on high for one minute, then stir. If it is fully melted, leave it to cool. If not, microwave for another minute on high then stir until fully melted. Then leave to cool.
In a large bowl add 130 g light brown sugar (or light Muscovado sugar) to 2 eggs. Whisk using a hand mixer for around 5 minutes or until it has increased in volume and is thick and foamy/creamy.
Check that the melted chocolate and butter is not still too hot from heating in the microwave. Pour the melted chocolate and butter into the whisked eggs and sugar. Fold in carefully.
Sift in 80 g plain flour and fold in carefully until fully mixed.
Crush half of the Mini Eggs (50 g) and add them to the brownie batter.
Line your slow cooker, I like to use two sheets of baking paper which I crumple then flatten again to make them pliable.
Pour in the brownie batter.
Put on the lid with a few sheets of kitchen towel or a tea/dish towel under the lid to catch drips.
Cook on high for 2 hours, or until the edges of the brownies are firm. At this point add the rest of the Mini Eggs (50 g) on top (they will sink in a little) and cook on high for another half an hour, or until the centre of the brownies is firm. If you like your brownies soft and fudgy you can take them out when there is still a slight wobble in the middle, this is up to you.
Remove the brownies from the slow cooker pot by using the edges of the baking paper. Leave them to cool if you can wait that long, then dive in!