Preheat the oven to 180°C (fan)/200°C/gas mark 6. Grease a 20cm square cake tin then line with non stick baking paper. Cut your paper a little longer than the tin so that you can use the ends to help you remove the brownie when cooked.
Break 200g dark chocolate into pieces and place in a heat proof bowl. Add 150g dairy free butter, chopped into pieces. Place over a small saucepan with a little hot water in over a low heat and melt, resisting the urge to stir too often.
You could also do this in the microwave but take care as the bowl will get very hot. Heat for 30 seconds, remove and stir, then return to the microwave for another 30 seconds. Repeat until melted.
Meanwhile place 2 tablespoons of ground flax seed in a bowl and add 4 tablespoons cold water. Stir and set aside to thicken.
Add a tablespoon of cider vinegar and a teaspoon vanilla extract to 100 ml oat milk. Stir and set aside.
In a large mixing bowl measure out 150g self raising flour and add 1 teaspoon bicarbonate of soda.
Mix well with a whisk then add 150g light brown sugar. Stir to mix and use your fingers to break up any lumps. Add a pinch of sea salt and mix.
When the chocolate has melted remove from the heat and set aside to cool for 1-2 minutes.
Pour the milk mixture and the flax egg mixture into the flour and gently stir to combine.
Carefully add the melted chocolate and gently fold in until everything is combined.
Pour your brownie batter into your cake tin.
Bake in the oven for 20 minutes then remove from the oven, sprinkle the mini eggs* over the top and return to the oven for 5-10 minutes.
The brownie should be firm but with a slight wobble to it and the mini eggs should have slightly sunk into the brownie mixture and begun to crack and melt.
Leave to cool completely in the tin before cutting. This will help it to hold together. For best results leave overnight or for at least four hours*.
Use the baking paper to help you remove the brownie, place on a board and cut into squares with a sharp knife.