First make the base. Blitz the biscuits in a food processor until you have fine crumbs.
Add the melted butter and pulse until thoroughly mixed.
Lightly grease (or spray with cake release spray) two 10cm (4 in) loose bottomed tart tins. Divide the crumbs in the tins and press down firmly using the back of a spoon to line the base.
Press the crumbs up the sides of the tins using your fingertips. Make sure you have a sturdy, even base and sides.
Chill for a couple of hours or overnight. When chilled remove from the tins very carefully as the sides can be a little fragile.
Make the tipsy cherries by putting all the ingredients in a small pot, bringing to the boil then simmering for 10 minutes. Sieve the cherries and reserve the liquid. Cool before using.
To make the yogurt chocolate mousse, pour 2 tablespoons of very hot water in a bowl and sprinkle the gelatine on top. Stir briskly to dissolve.
Heat the milk in a small saucepan. Take it off the heat once small bubbles appear around the edge of the pot and add the gelatine. Whisk together to combine.
Pour the hot milk over chopped chocolate leave for a couple of minutes. Stir together until completely melted. Set aside to cool.
Fold in the yogurt in the melted chocolate together with 3 tablespoons of the reserved cherry syrup.
Line the bottoms of the tart cases with the cooked cherries.
Transfer the mousse into a large pipping bag fitted with a star nozzle. Pipe the mousse over the cherries. Chill for half an hour.
Spoon a little cherry syrup over the tarts and decorate with fresh cherries and extra chocolate shavings (optional).