• 3/4 cup (1 1/2 sticks) unsalted butter, room temperature, plus additional for pan
• 7 tablespoons finely ground maple sugar, divided
• 1/4 teaspoon coarse kosher salt
• 1 1/2 cups all purpose flour
• 1/4 cup (about) bittersweet chocolate chips
• 2 teaspoons pure maple syrup (preferably Grade B)
• Test-Kitchen Tip
If maple sugar is coarse, grind it in a food processor until it resembles granulated sugar.
• Preheat oven to 300°F. Butter 9-inch-diameter tart pan with removable bottom. Using electric mixer, beat room-temperature butter, 6 tablespoons maple sugar, and coarse salt in large bowl until light and fluffy and sugar is dissolved. Add flour and beat just until blended. Pat dough evenly onto bottom of prepared pan. Press chocolate chips into dough in random pattern, spacing chips 1/2 inch apart (chips should show). Brush dough with maple syrup and sprinkle evenly with remaining 1 tablespoon maple sugar.
• Bake shortbread until golden brown and firm to touch, about 55 minutes. Transfer pan to rack and cool shortbread 10 minutes. Gently push tart pan bottom up, releasing shortbread. Cut warm shortbread into 16 wedges. Cool shortbread wedges completely and serve.