Preheat oven to 350 degrees F. Lightly spray an 8x8-inch baking dish with cooking spray. Line with a sheet of parchment paper, leaving an overhang on two sides.
To make the crust, place the white whole wheat flour, coconut sugar, and salt in the bowl of a stand mixer fitted with the paddle attachment. (Alternatively, the crust may be made in a food processor or even mixed by hand with a pastry blender.) Mix to combine. Add the butter cubes and mix on low speed until the butter pieces are small and evenly distributed. The mixture will still look crumbly but should hold together slightly when pressed between your fingers.
Reserve 1 cup of the crust mixture for the topping and press the rest of it into the bottom of the prepared baking dish. Bake for 14-15 minutes.
Meanwhile, make the peach filling. Place the ¼ cup white whole wheat flour, egg, Greek yogurt, honey, vanilla, cinnamon and salt in a large bowl. Whisk until smooth. Gently fold in the chopped peaches. Pour the mixture over the bottom crust in the pan.
Sprinkle on the reserved 1 cup topping.
Bake for 45-55 minutes, until filling is bubbly and topping is golden. For best results, let cool completely in the pan and then refrigerate at least 2 hours before cutting into bars. For easier slicing, carefully run a knife along the edges of the pan and then use the parchment overhang to transfer the bars to a cutting board.
Store leftovers in the refrigerator.