If you’re using pre-made coulis, proceed directly to step 4.
Blend the raspberries to a smooth purée, then strain through a fine sieve to remove the seeds.
I used frozen coulis that I had prepared beforehand.
Pour the coulis into a saucepan with the sugar and lemon juice. Mix well.
Mix the cornstarch with 2 tablespoons of water. Bring the saucepan to medium heat, and as soon as the coulis starts to heat up, pour in the diluted cornstarch while stirring.
Thicken the mixture like a custard. The curd gradually thickens and begins to boil.
Keep boiling until you obtain a thick consistency that coats the spoon well.
The curd will thicken further as it cools.
Remove from the heat and wait 1-2 minutes, stirring occasionally, then add the butter (if you wish to add it for extra creaminess).
Mix well until the butter is completely melted and incorporated into the curd.
Let cool slightly, stirring occasionally. Cover with cling film and refrigerate until ready to use. Wait at least 2 hours before using.