Preheat the oven to 180 degrees Celsius. Line a 20cm square cake tin with baking parchment.

Put the chocolate and butter in a bowl and put over a pan of boiling water. Stir regularly until it is melted. Remove from the heat and let it cool to room temperature.

Crack the eggs into a large bowl. Add the sugar and whisk until light, thick and creamy.

Tip in the cooled chocolate and butter. Use a spatula to fold it in in a figure of eight motion, being careful not to knock air out of the egg.

When it is fully mixed, add the flour and do the same. Stir in the blackberries, then pour into the prepared cake tin.

Cook for 25 minutes until firm. Remove from the oven and allow to cool fully in the tin (best done in the fridge) before slicing into 10 pieces.