Preheat the oven to 190°C, fan 170°C, gas mark 5, and line a 20cm square tin with baking parchment.
Melt the chocolate and butter in a heatproof bowl over a pan of simmering water, being careful not to let the bowl touch the water.
Whisk the eggs until pale and fluffy, then add the dark muscovado sugar and whisk until thick.
Gently fold in the melted chocolate and butter mixture.
Sift in the flour and fold in until the mixture is smooth, then stir in the glacé cherries (if using).
Pour the mixture into the prepared tin and bake for 30-35 minutes, until you see a paper-like crust on top - there should still be some movement in the centre of the tin.
Remove from the oven, leave to cool, then cut into squares.
*All information included in this recipe has been provided by Billington's.