Preheat oven to 375°F.

Wash, slice, and optionally peel the peaches.

In a large mixing bowl, combine sliced peaches, ¾ cup granulated sugar, lemon juice, cornstarch, vanilla extract, and cinnamon if using.

Stir gently and let sit for 10 minutes to release juices.

In a separate bowl, mix the flour, ½ cup sugar, baking powder, and salt.

Add cold cubed butter to the flour mixture.

Use a fork or pastry cutter to cut in the butter until the mixture is crumbly and resembles coarse sand.

Pour in ¼ cup boiling water and stir just until a thick dough forms.

Pour the peach filling into a 9-inch square baking dish or similar-sized pan.

Spoon the topping over the peaches in small mounds, leaving some gaps.

Flatten large mounds slightly with the back of the spoon.

Sprinkle coarse sugar on top if desired.

Bake for 38 to 42 minutes, until the topping is golden brown and the peach filling is bubbly around the edges.

Remove from oven and let cool for at least 15 minutes before serving.

Serve warm or at room temperature, plain or with vanilla ice cream or whipped cream.