1: Bring 400 g blackcurrant coulis (without sugar) to a boil, skim the foam that forms on the surface.
2: Add 100 g glucose syrup and 450 g caster sugar at once.
3: Bring to a boil again while whisking gently. You may need to skim again.
4: Mix in a bowl 50 g caster sugar and 40 g jam gelling agent, then add this mixture to the pan, whisking briskly. Note: We make this sugar + gelling agent mixture to facilitate the incorporation of the gelling agent into the blackcurrant, and to avoid that it clumps.
5: Bring to a boil again, then keep at a low simmer for 3 to 5 minutes.
6: Turn off the heat, skimming one last time if necessary.
7: Pour into a 20 x 20 cm (8 x 8 inch) metal frame, lined with a sheet of cling film. If you don't have this kind of frame, use a simple dish, like a casserole dish. Let cool, then put in the fridge for 2 hours or overnight.