Put the chocolate, butter and bourbon into a heatproof bowl, resting over a pan of simmering water. Heat for a few minutes until the chocolate has melted.
Stir in the sugars, mixing together well, then remove from the heat.
Beat in the eggs and vanilla extract, then fold in the flour and baking powder.
Pour the mixture into the prepared tin and bake for 40 minutes until the mixture is just firm.
Cool, then cut into squares and enjoy!
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