Preheat your oven to 350 degrees F. Line an 8-inch x 8-inch square pan with 2 pieces of parchment paper laid perpendicular to each other. Make sure the paper is long enough to hang over the sides. Spray the paper with non-stick cooking spray. Set aside.
Add the whole wheat flour, all-purpose flour, brown sugar, cornstarch, lemon zest, baking powder, and salt to the bowl of a food processor. Cover and pulse to mix.
Add the butter and yogurt to the food processor. Cover and pulse until the mixture turns into evenly sized-crumbs.
Dump the crust mixture into the prepared baking pan and spread into an even layer. Use the bottom of a measuring cup to press the crust into a firm layer (similar to a graham cracker crust).
Bake for 20-22 minutes until golden brown. Remove from the oven and cool the crust completely, about 1 hour.
Filling
Preheat your oven to 325 degrees F.
In a large mixing bowl, whisk together the eggs, honey, lemon zest, whole wheat flour, and salt until smooth and combined. Whisk in lemon juice and then pour the filling mixture over the cooled crust.
Bake for 21-23 minutes until the filling is set. Remove from the oven and allow to cool completely on a wire rack in the pan, about 1 hour. Chill in the fridge for 1-2 hours.
Once cooled completely lift the parchment paper to remove the dessert from pan. Cut the square into nine bars. Dust the entire surface liberally with powdered sugar before separating the bars.