Over medium heat combine rhubarb and sugar in a large saucepan stirring until the sugar coats the rhubarb slices.
Cover for five minutes stirring occasionally.
At this point the rhubarb will have started to break down and turn into liquid. Add the vanilla paste and the strawberry liquor if you are using it.
Turn heat to low, cover and let the mixture simmer for 30 minutes.
After 30 minutes turn the heat off. The mixture should be pink and thick with chunk of rhubarb. If your preference is a compote rather than a coulis you can leave it as is, letting it cool before jarring.
Allow the mixture (compote) to cool for anywhere between 10 and 30 minutes.
Once somewhat cooled pour the compote into a blender and blend until smooth.
Pour mixture back into the saucepan and place back on the stove on low heat for 10 minutes to thicken. If your coulis is already thick enough straight from the blender then all you want to do is bring the coulis back to a simmer.
Add the butter, stirring to melt and combine, giving the coulis a gloss.
Allow to cool and jar or freeze the coulis.