• Preheat the oven to 350°F, and adjust a rack to the top third.
• Use a tiny bit of the 1 teaspoon of butter to lightly dab the bottom and sides of an 8 X 8 inch baking pan. Now line the pan with parchment paper (the dabs of butter will help it stick). Be sure to use a large enough piece that will cover the bottom and sides. Cut slits in each corner so that it will lay flat. Now grease the parchment paper with what's left of the 1 teaspoon of butter. Set aside.

For the crust

• In a large mixing bowl, combine the butter and sugar. Use an electric mixer or a standing mixer with the paddle attachment, to cream it together until it's light and fluffy, about 5 minutes.
• Add the coconut flour and gluten-free all-purpose flour, and beat on a low speed just until it's completely incorporated.
• This will be a relatively wet dough. Use your hands to scoop it into the parchment-lined pan, and then use your hands to gently press it into the bottom.
• Bake in the preheated 350°F oven just until it's becoming golden, about 15 minutes. Remove it from the oven and let it cool at room temperature.

For the lemon topping

• While the crust is baking, in another large mixing bowl, whisk the egg with the egg yolks and sugar until it's smooth, thick, and lighter in color -- about 4 minutes. Add the lemon zest and juice and whisk until it's blended.
• Pour the filling over the baked, cooled crust (it's okay if it's still a bit warm). Return the pan to the oven and bake just until it's set and lightly browned on top, about 25 minutes. (If you fear it's becoming too brown and isn't yet set, lay a piece of foil lightly over the top until it's done.) Remove from the oven and let it cool completely in the pan.
• Once it's cool, very gently and carefully, lift it out of the pan with the parchment paper, and place it on a clean, dry surface where it can be sliced. Slice the lemon bars into nine squares, and place one lemon slice on the top center of each one.
• Now add the powdered sugar to a small mesh strainer and gently tap its side, to sift the sugar over all of the bars. It will almost instantly dissolve on the lemon slices, while the corners and sides will stay white. (You will likely not use all of the sugar.)
• To store the lemon bars, place them in an airtight container separated by parchment or wax paper layers, in the refrigerator.