In a medium mixing bowl, combine all purpose flour, baking powder, salt, and cornstarch. Whisk well to thoroughly combine. Set aside.
In the bowl of a stand mixer (or, in a mixing bowl with a handheld electric mixer), cream butter and sugars until light and fluffy, 3 - 4 minutes.
Add egg and beat to combine. Scrape down sides and bottom of the bowl. Add maple syrup, maple extract and vanilla extract. Beat to combine.
Add flour mixture to wet ingredients. Beat on low speed until most of the flour is incorporated, then beat on medium speed until the mixture is uniform, about 3 minutes. Using a rubber spatula, fold cookie dough from the bottom of the bowl over the top, to ensure everything is well combined, then beat for an additional minute.
Remove dough from bowl with a sturdy rubber spatula. Press into a rectangle, wrap tightly with plastic wrap and refrigerate for 30 - 45 minutes.
Roll out dough to 1/8" (1/2cm) thickness on a lightly floured surface, or between two sheets of parchment paper. Using a small maple leaf cookie cutter, cut out cookies. Place the cut out cookies on a parchment-lined baking sheet and return to the fridge.
Preheat oven to 325°F with convection fan // 350° F without fan // 180°C without fan // Gas mark 4.
Place cookies on parchment-lined baking sheets (or use a silicone baking mat), leaving 1 inch between each one, and bake for 10 - 12 minutes. *Bake one sheet at a time for best results. Keep cookie dough in the fridge until ready to bake.
Remove from oven and let cool on baking sheet for 5 minutes before moving to cooling rack to cool completely. While cookies are cooling, make maple buttercream (recipe below).
When cookies have cooled completely, line them up in pairs, flipping over the bottom of the sandwich cookie.
Using a piping bag, or a small spatula, fill cookies with 1 1/2 - 2 teaspoons of buttercream.
Place top cookie on buttercream and lightly press to secure in place.
Store in an airtight container at room temperature for 3 days, or refrigerate for up to 7 days.
Maple buttercreamCombine all ingredients for maple buttercream in a medium mixing bowl and beat on medium/medium-high speed for 3 - 4 minutes, until light and fluffy.