Preheat the oven and prepare the pan: Place a rack in the center of the oven and preheat the oven to 375°F. Grease the bottom and sides of an 8x8-inch square pan with butter. Line the bottom and two sides of the pan with a piece of parchment paper, leaving an overhang on both sides.

Make the crust and crumb topping: In a large bowl, stir together the flour, sugar, salt, and cinnamon. Add in the cold, cubed butter and the vanilla extract, and use your fingers to work the butter into the dry mixture until it holds together when squeezed. Set aside ¾ cup of the mixture to be used for the crumb topping. Transfer the remaining mixture to the pan and press into an even layer. Set aside while you prepare the peach filling.

Make the peach filling: In a medium bowl, whisk together the egg, sour cream, sugar, flour, vanilla extract, cinnamon, and salt to combine. Add the sliced peaches to the mixture and toss to combine. Spread the peach filling evenly over the crust.

Add the crumb topping: Evenly sprinkle the reserved ¾ cup crumb topping over the peach mixture. To form bigger crumb pieces, squeeze the mixture in your hand before sprinkling. Most of the pieces should be roughly pea-sized.

Bake the bars: Bake until the crumble is a pale golden color and feels crisp, about 50 minutes. Place the pan on a wire rack and allow to cool for at least 1 hour before slicing and serving. The bars will keep in an airtight container at room temperature for up to 5 days or in the refrigerator for up to a week. Once cut into squares, these bars freeze well for up to 3 months. Love the recipe? Let us know with a rating and review!