Preheat oven to 350°F (175°C). Line a 9x9 inch metal baking pan with parchment paper, leaving an overhang on two sides for easy removal.

Prepare the crust/topping mixture: In a medium bowl, whisk together the flour, granulated sugar, baking powder, and salt. Cut in the cold, cubed butter using a pastry blender or your fingers until the mixture resembles coarse crumbs.

Press about two-thirds (2/3) of the crumb mixture firmly into the bottom of the prepared baking pan to form the crust. Reserve the remaining one-third (1/3) for the topping.

Prepare the filling: In a separate bowl, gently toss the sliced nectarines with the brown sugar, cornstarch, lemon juice, and cinnamon until evenly coated.

Spread the nectarine filling evenly over the crust layer in the pan. Sprinkle the reserved one-third of the crumb mixture evenly over the fruit filling.

Bake for 45 to 50 minutes, or until the topping is golden brown and the filling is bubbly. Let the bars cool completely in the pan on a wire rack (at least 2 hours) before lifting out using the parchment overhang and slicing into squares.