1) Preheat the oven to 375°F.
2) To make the cookies: Beat the butter, maple sugar, salt, vanilla, and maple flavor until fluffy.
3) Stir in the flour, cornstarch, and nuts. Wrap the dough, and refrigerate it for 30 minutes.
4) Press 1 teaspoonful of chilled dough into each mold. Place the molds on a baking sheet.
5) Bake the cookies for 10 to 12 minutes, until they're browned around the edges. Cool the cookies for a few minutes, then push on the thick edge of each one to pop it out of the mold. Repeat with the remaining dough.
6) To make the filling: Whisk all the ingredients together, adding as much liquid as necessary to make a thick, spreadable filling.
7) Spread a 1/8" to 1/4" thick coating of filling on the flat side of each cookie; press together with another cookie. Allow the filling to set completely before serving.
Yield: 18 to 20 sandwich cookies.