Dissolve the espresso powder and liqueur in a bowl and set aside.
Place the egg yolks and half the sugar in a medium bowl, whisk together until pale and about three times the volume.
In a large bowl, whip the cream and other half of the sugar until it forms soft-medium peaks. Add the mascarpone and whip until it forms medium peaks.
Carefully fold in the sweetened egg yolks to the mascarpone-cream mixture.
Use a small sieve to dust an 8in square baking other deep sided dish with ½ tablespoon cocoa powder
Carefully and quickly dip a sponge finger into the reserved espresso liquid, and place into the dish. They soak up liquid quickly, so be careful! Repeat until all the surface area of the dish is covered.
Dollop ⅓ of the mascarpone mix over the soaked sponge fingers, and repeat the layering process.
Finish with a top layer of mascarpone mix - this can be spread on with a spoon, or piped on with a piping bag. Once covered, dust the top of the tiramisu with cocoa powder, and grated chocolate flakes.
Cover with cling film and allow to chill for a minimum of 30mins, and preferably for up to 4 hours - this encourages the flavours to infuse, and for the biscuits to properly absorb the liquid.