Preheat oven to 350°F. Lightly grease a 9x13-inch ceramic or glass pan. Do not use metal; it can cause the fruit to have a metallic flavor.
Filling: Prepare the peaches (see note 1).Add chunks of peaches to a large bowl. Toss gently with lemon juice. Add dark brown sugar, granulated sugar, cinnamon, nutmeg, cornstarch, salt, and vanilla. Stir gently until combined, then transfer to the prepared pan along with any accumulated juices. Bake filling (without topping) for 10 minutes.
Crumble Topping: Melt the butter and allow plenty of time for it to come back to room temperature. If it is too hot, it will melt the sugar and make the topping greasy. In a large bowl, combine flour, baking powder, salt, sugar, and vanilla; stir. Add room-temperature melted butter and stir until just combined. Do not overmix. Crumble with your fingers and sprinkle over the partially baked peaches. Do not press the crumble mixture down. Return to oven for 35 more minutes.
Crunchy Topping: In a small bowl, stir together the sugar and cinnamon. Sprinkle evenly on top of the crumble topping. Return to oven for 10 minutes. If desired, broil for 2–3 minutes on high for a super crisp topping. Remove from oven and let stand 20–30 minutes to thicken and gel. Serve with fresh whipped cream or vanilla bean ice cream (see note 2).
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Notes
Note 1: Peach prep:
Use a paring knife to cut off the skin; halve the peaches, remove the pit, and chop.
Briefly boil (blanch) the peaches:
Unripe peaches: Using a paring knife, score a small “X” in the bottom of the peach, then place the peach in boiling water for about 60 seconds. Using a slotted spoon, remove the peach from the boiling water and place it in a large bowl filled with ice and cold water. As soon as the peach is cool enough to handle, use a paring knife to remove the softened skin (it should slide off pretty easily).
Ripe peaches: Place the peach in boiling water for about 30 seconds. Using a slotted spoon, remove the peach from the boiling water and place it in a large bowl filled with ice and cold water. As soon as the peach is cool enough to handle, gently peel off the softened skin with your fingers (should slide right off).
Note 2: Broiling the Peach Cobbler at the end of the baking time makes it crispy and crunchy. Use caution, though, depending on the type of pan you use. Some types are not suitable for the broiler.Storage: Store cooled peach cobbler in the fridge for up to 3–4 days. Before baking, keep prepared topping, filling (without peaches), and chopped peaches in separate airtight containers in the fridge for up to 1 day.