Cookies: In medium bowl, whisk together flour, cornstarch, baking powder and salt.
Step 2
In large bowl, using handheld electric mixer, beat together butter and sugar until light and fluffy. Beat in egg and vanilla until incorporated.
Step 3
On low speed, beat in flour mixture just until combined. Do not overbeat, remove from bowl. Divide dough in half and shape each half into a disk. Wrap each disk with plastic wrap and chill for 45 to 60 minutes or until dough is chilled and firm.
Step 4
Preheat oven to 350°F (180°C). Working with one portion at a time, roll dough on lightly floured work surface to 1/4-inch (0.5 cm) thickness (if dough is too hard to roll, let stand at room temperature for 5 to 10 minutes).
Step 5
Using 2 1/2-inch (6 cm) round cookie cutter, cut out 44 to 48 rounds, rerolling scraps as needed. Using 1-inch (2.5 cm) holiday-themed cookie cutter, cut out a window in centre of half the cookies. (Bake cutout cookies or reroll.) Transfer cookies to parchment paper–lined baking sheet, spacing about 2 inches (5 cm) apart. Freeze for 10 minutes.
Step 6
Bake cookies for 10 to 14 minutes or until edges are lightly golden and bottoms are set. Let cool on pan for 5 minutes, then transfer to wire rack to cool completely.
Step 7
Glaze: In bowl, whisk together confectioners’ sugar, milk, honey or maple syrup and vanilla until smooth.
Step 8
Dip window cookies in glaze to coat one side, letting excess drip back into bowl. Arrange on parchment paper–lined sheet and let stand for 1 to 2 hours or until hardened and set. Spoon 1/2 tsp (2 mL) jam on remaining cookies, spreading slightly; place glazed cookie on top. Repeat assembly until complete.