Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, whisk together the flour, sugar, salt, and baking powder. Add the softened butter, egg, and vanilla extract. Mix until the dough is crumbly but holds together when pressed.
Press half of the dough mixture into the bottom of the prepared baking dish to form a crust.
In another bowl, combine the sliced peaches, lemon juice, cinnamon, and cornstarch. Toss until the peaches are evenly coated.
Spread the peach mixture over the crust in the baking dish. Crumble the remaining dough over the top of the peaches.
Bake for 35-40 minutes, or until the top is golden brown and the peaches are bubbly.
Allow the bars to cool in the pan for at least 30 minutes before slicing into squares.