In the bowl of a stand mixer fitted with a paddle attachment (or with a large bowl and a handheld electric mixer), cream together the butter and brown sugar until it is light and fluffy.
Beat in the maple syrup and maple extract (if using).
In a medium bowl, whisk together the flour, cornstarch, and salt, then gradually add the flour mixture to the butter mixture just until evenly mixed.
Wrap the dough in cling wrap and refrigerate for at least 2 hours and up to 2 days.
When ready to bake, preheat the oven to 350°F (180°C) and line 2 baking sheets with parchment paper.
On a well-floured surface, roll out half of the dough to ¼-inch (6-mm) thick, leaving the rest in the refrigerator to stay cold.
Cut out cookies with a 2-inch (5-cm) maple leaf cookie cutter (or shape of your choice) and transfer to the prepared baking sheets, leaving about 1 inch (2½ cm) of space between.
Gather and reroll scraps until the dough becomes too soft to work with. Return this dough to the fridge to chill while you work with the chilled half.
Bake for 12-15 minutes, until the cookies start to turn a pale golden brown around the edges.
Let cool on a wire rack while you bake the remaining cookies.
Store in an airtight container at room temperature for up to 3 days.