Melt the butter in a pan and stir in the biscuits and 50g/2oz of the pecans. Press into the base of a lightly greased 18cm/7in round fluted flan tin, then chill in the fridge.
Melt 175g/6oz of chocolate in a bowl placed over a pan of gently simmering water, cool slightly. Whip the cream until stiff. Beat half of the mascarpone in a bowl together with the coffee, sugar and melted chocolate. Gently fold in the cream. Pour into the flan tin and smooth the top; chill until firm.
Meanwhile, melt the remaining chocolate over a pan of gently simmering water and leave to cool slightly. Transfer the cheesecake to a serving plate. Mix together the remaining mascarpone and the yogurt and swirl over the top of the cheesecake. Drizzle over the chocolate, sprinkle over the remaining pecans and dust lightly with cocoa powder.