PREHEAT oven to 350°F.
BUTTER and flour one 11? inch tart pan.
MELT butter 16 oz. of chocolate together in a heatproof bowl. You can either melt the chocolate by placing the bowl over a pot of simmering water, or by microwaving it in 30 second bursts. Stirring after each interval. Make sure butter and chocolate melt and are stirred together completely – then set aside to cool. In an electric mixer beat the eggs, sugar, coffee, hazelnut and vanilla until light and fluffy. Add melted, cooled chocolate and mix to combine.
COMBINE flour, baking powder, salt and remaining 4 oz. of chopped chocolate in a separate bowl. Gently fold this into the wet ingredients until just combined.
POUR batter into prepared tins and bake for 35-40 minutes, or until the center is puffed and crackly on top.
COOL before removing from tart pan.