Using an electric mixer, beat butter with one egg until smooth.
Add almond flour, sugar, all-purpose flour, and lemon juice, and beat with the mixer on the lowest setting until completely combined and smooth.
Shape the dough into two logs, about 2 inches each in diameter; cover logs in plastic wrap and refrigerate for 3 hours.
Preheat oven to 350˚F.
Slice each log into cookies, about 1/8-inch thick, and place on a lightly greased baking sheet.
Lightly beat the second egg and brush the top of each cookie with it.
Place one lemon slice on top of each cookie.
Sprinkle with superfine sugar and bake for 12 to 15 minutes.
If you like the tops of the cookies browned, put them under the broiler the last 3 minutes of baking.
Cool and optionally sprinkle with powdered sugar.