Preheat the 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, combine the flour, granulated sugar, salt, and baking powder. Stir until well mixed.
Add the melted butter, egg, and vanilla extract to the dry ingredients. Mix until a crumbly dough forms.
Press about two-thirds of the dough mixture into the bottom of the prepared baking dish to form a crust.
In another bowl, combine the diced peaches, lemon juice, ground cinnamon, and brown sugar. Toss gently to coat the peaches.
Spread the peach mixture evenly over the crust in the baking dish. Crumble the remaining dough over the top of the peaches.
Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the peaches are bubbly.
Allow the bars to cool in the pan for at least 15 minutes before slicing into squares. Serve warm or at room temperature.