Preheat the oven to 180°C (350°F) and line a baking pan with parchment paper.
In a large bowl, mix together the flour, granulated sugar, brown sugar, baking powder, and salt.
Add the butter, egg, and vanilla extract, mixing until a crumbly dough forms.
Press two-thirds of the crumb mixture evenly into the prepared baking pan.
In another bowl, combine the diced peaches, sugar, cornstarch, lemon juice, and cinnamon.
Spread the peach filling evenly over the crust.
Crumble the remaining dough evenly over the peach filling.
Bake for 35–40 minutes, or until the top is lightly golden.
Cool completely before slicing into bars and serving.
Notes
Allow the bars to cool completely before slicing for clean edges. Store leftovers in an airtight container for up to 3 days or refrigerate for up to 5 days.