1. Preheat the oven to 160°C (140°C fan-forced). Grease and line a deep 22cm round cake tin with baking paper.
2. Melt the butter, dark chocolate, coffee and milk together over low heat in a small saucepan. Stir until smooth. Remove from heat and cool.
3. Sift flour and cocoa into a large bowl, add sugar and mix well. Fold in chocolate mixture with eggs, vanilla and sour cream. Stir until combined.
4. Pour into prepared tin and bake for 1 hour 10 minutes or until the cake is cooked when tested with a skewer.
5. Cool on a rack for 15 minutes before inverting onto a serving plate.
6. Meanwhile, make mint julep syrup by combining sugar and 1 cup (250ml) water in a small saucepan. Stir over low heat to dissolve the sugar only until it comes to the boil. Simmer, without stirring, for 10 minutes or until thick and syrupy. Remove from heat and pour into a small heatproof bowl. Cool. Stir through mint leaves, lemon juice and Bourbon.
7. Serve wedges of mud cake with crème fraiche or whipped cream drizzled with mint julep syrup.