Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper for easy removal.
In a large bowl, whisk together the flour, sugar, salt, baking powder, and baking soda.
Add the softened butter, egg, and vanilla extract to the dry ingredients. Mix until a crumbly dough forms.
Reserve 1 cup of the dough for the topping. Press the remaining dough evenly into the bottom of the prepared baking dish.
In another bowl, combine the sliced peaches, lemon juice, cornstarch, and cinnamon. Toss until the peaches are well coated.
Spread the peach mixture evenly over the crust in the baking dish.
Crumble the reserved dough over the peach filling.
Bake for 35-40 minutes, or until the top is golden brown and the filling is bubbly.
Allow the bars to cool in the pan for at least 30 minutes before slicing into squares.