Put the flour, baking powder, cocoa powder and sugar into a large bowl. Add the butter and using your fingertips rub the butter into the dry ingredients.
Continue rubbing until the mixture forms coarse breadcrumbs.
Make a well in the centre, add the egg yolk and water, lightly knead just until the dough is smooth and forms a ball. Do not overwork it.
Place the dough in the fridge and chill for at least an hour. Use this time make the honey ganache.
Place the honey and the whipped cream into a heavy based saucepan and bring to the boil.
Put the chopped chocolates together into a bowl that is big enough to fit the cream.
Add the hot cream to the chocolate and stir until the chocolate has melted, then stir in the butter, continue to stir until the butter melts and is combined. Put the ganache to one side to cool.
Preheat the oven to 180C Fan and roll out the pastry between two sheets of greaseproof paper to about 4mm thick.
Stamp out rounds of pastry that are slightly bigger than the cavities you're filling, using a pastry tamper, press the pastry into the holes, using a fork, prick one set of holes in the bottom of the pastry and place the tray in the fridge for 10 minutes.
Bake the pastry for 10-12 minutes until crisp, leave to cool in the tins.
When the pastry is cool, pour or spoon the ganache into the pastry cases, you want the ganache to be quite runny so that it has a smooth surface when poured into the pastry. If your ganache has set too much warm it in the microwave in bursts stirring in between, you want the consistency of double cream, don't let the ganache burn if you do this.
Leave the chocolate tarts to set and make the crystallised cobnuts. Preheat the oven to 180C Fan.
Peel and shell the cobnuts, roast in the oven for 10 mins. If any of the brown skins become loose remove them, by rubbing them gently when they've cooled.
Put the sugar and water in a saucepan and cook to 118C, add the cobnuts to the sugar and mix well. Leave the pan on the heat and stir the nuts continuously until the sugar crystallizes.
Remove the nuts from the heat and spread them out onto a non-stick silicone baking mat. Make sure the nuts are separate and not stuck together. (These can be stored in an airtight container and used within 2 weeks.
Top the tartlets with a cobnut and leave for a couple of hours to set. These will last for a week in an airtight container and they don't need to be stored in the fridge - unless it's really hot.