Preheat the oven to 375°F (190°C).
Place the blackberries in a small baking dish with a ¼ cup of granulated sugar, lemon juice, and cinnamon. Stir gently to combine.
Combine the flour, baking powder, and salt in a bowl. Make a well in the center, then pour in the cream and add the lemon zest. Knead until a dough forms.
Pat the dough into a 6-inch square, and then cut it into nine squares. Scatter the dough pieces on top of the blackberries.
Brush the dough with the egg yolk, if using.
Bake for 20 minutes, until the berries are bubbling and the biscuits are browned. Serve warm.
