Heat the oven to 350 degrees F. Line a 9 by 9 inch square baking pan with parchment paper, leaving overhang on two opposite sides so the bars lift out easily.

In a large bowl, stir the melted crust butter with the granulated sugar and 2 teaspoons vanilla until blended. Add the 2 1/2 cups flour and salt, mixing until a soft dough forms. Press the dough evenly over the bottom of the prepared pan, then bake for 15 minutes until the edges just barely turn golden.

While the crust bakes, toss the diced peaches with the light brown sugar, 2 tablespoons granulated sugar, cornstarch, cinnamon, and 1 teaspoon vanilla. Spread the peach filling evenly over the warm crust.

In another bowl, mix the oats, 3/4 cup flour, 1/2 cup brown sugar, cinnamon, and sea salt. Pour in the 6 tablespoons melted butter and stir until crumbly clumps form. Sprinkle the crumble evenly over the peach layer, then return the pan to the oven and bake for 30 minutes until the topping is golden and the fruit is bubbly.

Cool the bars completely in the pan. Whisk together the powdered sugar, milk, and remaining 1/2 teaspoon vanilla to make a smooth glaze, then drizzle it over the cooled bars before cutting into squares.