1. Put the mascarpone cheese, sugar and egg yolks into a bowl and beat until smooth. Add the chestnut puree and beat again until completely smooth.
2. Whisk the egg white until standing in stiff peaks. Fold 1tbsp of the egg white into the chestnut mixture and once combined, gently fold in the remaining egg white.
3. In a shallow dish mix the coffee with the liqueur. Take one of the sponge fingers, break into pieces to fit the size of the serving dish, and dip into the coffee mixture. Place into the bottom of the serving dish. Add a few pieces of strawberry and then spoon over about 2tbsp of the chestnut mixture. Repeat the layers using two sponge fingers. Make up three servings in the same way.
4. Chill the tiramisu until ready to serve. For best results, chill overnight to let the flavours mingle.
5. Before serving dust the top of each dessert with sifted cocoa powder.
Variation: Try using other seasonal fruit such as a thin slice of peach or nectarine instead of strawberries.