Preheat the oven to 400°F (200°C). Grease a 1.5QT (1.5L) heat-resistant pan with butter.

Filling: Place the cornstarch and cold water in a large bowl and mix well. Add the blackberries, granulated sugar, lemon juice, and lemon peel. Mix together well. Once combined, place into a thick, enamel pot.

[Make the biscuits] Add the all purpose flour, 1/4 cup granulated sugar, baking powder, lemon peel, and salt into a medium sized bowl. Use a whisk to stir together while allowing air to mix in.

Cut the cold butter into 1 cm cubes and add to Step 3. Use a pastry blender or 2 dinner knives to mix the butter into the powder using cutting motions (do not knead). Once it becomes fine and crumbly, add the hot water and use a spatula to gather the dough together. (The dough should be sticky.)

Bake for 20-25 minutes in the preheated oven. (Bake until the biscuit dough becomes nicely browned.) If your pan is like the picture and there isn't much space left on the top, there is a chance that it will overflow, so it's best to place a piece of parchment paper on the oven rack underneath the pan.