2 ½cupsall-purpose flour, spooned and leveled,divided
⅓cupdark brown sugar,lightly packed
¾tspcoarse Kosher salt(if using table salt, use half the amount)
Peel the peaches and chop the fruit into 1/2 inch chunks. In a saucepan, combine the peaches, lemon juice and sugar. Over medium heat, bring the peaches to a boil, then continue to cook over medium low heat, stirring frequently, until the liquid becomes thick and syrupy. You should reduce the filling to about 1 1/2 cups.
Remove from the heat and stir in the vanilla extract. Let cool to room temperature, or make up to 3 days in advance and refrigerate until needed.
Crust and Topping
Preheat the oven to 350 F, and line an 8 inch baking pan with parchment paper, or spray with nonstick spray. Position an oven rack in the center of the oven.
Place the butter in a saucepan, and melt over medium heat. Continue to cook the butter, swirling the pan occasionally, until very fragrant and nutty brown solids form on the bottom of the pan. This should take about 5 minutes.
Meanwhile, combine 2 1/4 cups of the flour with the brown sugar, salt, baking powder, cinnamon and nutmeg. Pour the hot, browned butter over the flour mixture and stir into a soft dough.
Press 2/3 of the dough into an even layer in the prepared pan. Bake for 20 minutes.
Add the remaining 1/4 cup flour to the remaining 1/3 of the dough, and use your fingers or a fork to make a crumbly topping.
After the crust bakes for 20 minutes, take the pan out of the oven. Spread the peach filling over the hot crust, then sprinkle the filling with the crumble topping. Return the pan to the oven, and bake for an additional 35 minutes, until the filling is bubbling and the topping is golden brown.
Let the bars cool at room temperature for at least 2-3 hours before cutting into squares. Or, enjoy them warm, although they'll be a bit messier.
Leftover bars should be cooled completely, then stored in an airtight container at room temperature for up to 2 days.