Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

In a medium saucepan, combine the topped and tailed gooseberries and 3/4 cup granulated sugar. Cook over medium heat, stirring occasionally, until the gooseberries begin to soften and release their juices, about 8-10 minutes.

In a small bowl, whisk together the cornstarch and cold water until smooth. Pour the cornstarch slurry into the simmering gooseberries, stirring constantly. Continue to cook for 1-2 minutes, until the filling thickens.

Remove from heat and stir in the lemon juice and vanilla extract. Pour the gooseberry filling into the prepared baking dish and spread evenly. Set aside.

In a large mixing bowl, whisk together the flour, 1 tbsp granulated sugar, baking powder, and salt.

Add the cold, cubed butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining.

Gradually add the cold buttermilk, stirring with a fork until just combined. Do not overmix; the dough will be shaggy.

Turn the dough out onto a lightly floured surface and gently knead 2-3 times, just enough to bring it together. Pat or roll the dough to about 1/2 inch thickness.

Using a 2-inch biscuit cutter (or a knife), cut out approximately 12-14 biscuits. Reroll scraps if necessary.

Arrange the biscuits on top of the gooseberry filling in the baking dish, leaving a small space between each biscuit.

In a separate medium mixing bowl, combine the flour, granulated sugar, brown sugar, and cinnamon (if using).

Add the cold, cubed butter. Using a pastry blender or your fingertips, cut the butter into the dry ingredients until coarse crumbs form. Sprinkle the crumble mixture evenly over the biscuits.

Bake in the preheated oven for 40-45 minutes, or until the biscuits are golden brown, cooked through, and the crumble is deeply golden. The gooseberry filling should be bubbly.

If the crumble is browning too quickly, you can loosely tent the casserole with aluminum foil for the last 10-15 minutes of baking.

Let the casserole cool for at least 15-20 minutes before serving. This allows the filling to set slightly.