The filling for this subtly sweet and tangy tart is based on a blackberry panna cotta recipe found here: https://honest-food.net/foraging-recipes/sweets-and-syrups/blackberry-panna-cotta/. The crust comes from the wonderful cookbook Soulful Baker, by Julie Jones.
Toward the end of the 30 minutes, place the 4 tbsp cold water in large, spouted container. (I used a 4-cup capacity Pyrex measuring cup.) Sprinkle the powdered gelatin over the cold water.