Preheat the oven to 170'C /325'F /Gas Mark 3. Line a baking tray with greaseproof paper.
Melt the chocolate in a glass bowl over a pan of simmering water until it is completely melted. Remove from heat and put aside.
Sieve the flour into a large bowl and add the salt, baking powder and cocoa powder.
Beat together the butter and sugar, then add the egg and egg yolk, melted chocolate and vanilla essence. Add the mixed dry ingredients and knead the dough with your hands. If it is too sticky, add a little more flour.
Roll out the dough with a rolling pin until about 0.5cm thick. With a rectangular cookie-cutter, cut out as many rectangles as you can and place them on a sheet of baking paper. You may need to reform the dough and roll out a few times to get the maximum number of pieces. Place in a refrigerator for one hour.
Place in the oven for 12 to 15 minutes. Take out of the oven and cool the biscuits on a wire rack.
Cut slices of ice cream around the same size as the biscuits and 3cm thick. Place the ice cream on a biscuit. Put one tablespoon of Blackcurrant Conserve on it, then close the sandwiches with another biscuit. Continue until all the biscuits are used and serve immediately.