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Heat oven to 180C/160C fan/gas 4. Oil and bottom-line a 20cm round (6cm deep) loose-bottom tin. For the cake, mix the coffee with 2 tsp warm water and set aside. Tip the flour into a large mixing bowl. Stir in the baking powder, ground almonds, both sugars (use fingers to rub out any lumps from the muscovado) and walnuts, then make a dip in the centre. Put the eggs, yogurt, oil and coffee mix into the dip and stir the mixture with a wooden spoon so everything is evenly mixed. Spoon the mixture into the tin, smooth the top to level it, then bake for 40-45 mins, or until a skewer inserted into the centre of the cake comes out clean. Let the cake cool in the tin briefly, then turn it out and peel off the lining paper. Leave to cool completely while you make the filling and icing.
Make the syrup for the filling: put the caster sugar and coffee into a small, heavy-based saucepan, then pour in 3 tbsp water. Heat gently, stirring to help the sugar dissolve. Once dissolved, raise the heat, then boil at a fast rolling boil for about 2½-3 mins until thickened and syrupy. Pour into a small heatproof bowl and set aside to cool. When cold it should be the consistency of treacle.
Meanwhile, finish off the filling and make the icing. Beat the mascarpone, quark, icing sugar and vanilla together until smooth, then stir in the cold coffee syrup. Set aside. For the icing, sift the fondant icing sugar into a bowl. Mix the coffee with 1 tbsp warm water, stir this into the fondant icing sugar with about 1 more tsp water to give a smooth, thick but spreadable icing. Split the cake into 3, then sandwich back together with the filling. Spread the icing over the top, scatter over the chopped walnuts and leave to set. Store in the fridge.