In the bowl of an electric mixer, fitted with the paddle attachment, cream the butter and the maple sugar for several minutes.
Add the egg yolk, and mix to combine, scraping down the bowl as needed.
Stir in the flour and salt with the mixer on low, scraping the bowl as needed so that the butter mixture is incorporated with all the flour. Continue mixing until the dough holds together when pressed.
Divide the dough into two equal disks. If your kitchen is warm, you may have to chill the dough for 30 minutes at this stage to help the butter firm up. If so, wrap both disks in plastic wrap to chill. Otherwise, only wrap half in plastic wrap to prevent drying while you work with the rest.
Roll out the first disk of dough on a lightly floured surface until it's ⅛" thick. Cut into the maple leaf shapes with a 2¼ inch cookie cutter. Re-roll the scraps. Transfer all the cookies to a parchment-lined sheet pan. You should be able to fit about 15 per sheet, and between the two disks of dough, you will likely cut out about 56 maple shortbread cookies (to make 28 sandwich cookies).
Refrigerate for 15 minutes before baking while you preheat the oven.
Preheat the oven to 350°F (175 °C).
Once the cutout cookies are cold, using a pairing knife, score the cookies to create the vein patterns of the leaves on the surface of the cookies. This is optional but makes the leaf cookies more realistic, just like the commercial cookies. Don't cut through the dough, just score the surface of it.
Bake until the edges begin to brown lightly, 13–15 minutes, rotating if necessary partly through the baking time so that the edges brown evenly.
Let cool completely on a wire rack before sandwiching the cookies with filling.
Maple cream filling
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter until it is creamy and smooth.
Add the maple butter and maple sugar, and continue beating the mixture until it is light, fluffy, and smooth. This takes several minutes so be patient. Make sure to scrape down the bowl sides periodically to make sure all the ingredients are properly mixed. Use immediately.
Assembly
When the cookies have cooled and the maple cream cream filling is ready, spread a teaspoon of the filling on the underside of a maple leaf cookie, and top with another, pressing lightly to glue the two cookies together. You can use an offset spatula to scrape any excess filling.
The filling will be soft on the day you assemble the cookies and then will dry out slightly and set in place after about a day. Store the cookies in an airtight container in a cool, dry place.