Makes 2 1/2 – 3 1/2 dozen cookies.
Beat the butter using electric beaters or in the bowl of a stand mixer fitted with the paddle attachment on medium speed for a minute, to make it fluffy. Add the icing sugar and vanilla bean seeds and mix, starting on low but increasing to medium, for another minute. Sift in the flour, cornstarch and salt in and mix on medium-low speed until the dough comes together. Shape this into a disc, wrap in plastic wrap and chill for at least 2 hours before rolling.
Preheat the oven to 300ºF (150ºC). Line 2 baking trays with parchment paper. On a lightly floured surface, knead the dough just a little to soften it (this will prevent cracking as you roll) and then roll it out into a circle just under 1/4-inch (6 mm) thick. Use a cookie cutter to cut out desired shapes and then use a smaller cookie cutter (it can be a different shape) to cut out a hole in the centre of each cookie and then place them onto the baking tray, leaving an inch (2.5 cm) between cookies. Any scraps can be re-rolled.
Crush the hard candies by putting each colour in a resealable bag and crushing with your rolling pin. Sprinkle a generous amount of the crushed candy into the hole of each cookie, but be sure to brush away any candy bits that fall onto the surface of the cookie. Bake the cookies for 18-20 minutes, until they just begin to show signs of colouring at the edges. Let the cookies cool on the tray on a rack and then remove to enjoy.
For a decorative option, you can pipe detail with royal icing. Whisk 1 cup (130 g) of sifted icing sugar with 1 tsp (5 mL) of meringue powder and 1 Tbsp (15 mL) or tepid water until smooth. Pour this into a piping bag with a small plain tip and pipe dots or swirls as you wish. Let the icing sit for 2 hours to set before packing in an airtight container.
The cookies will keep for up to 5 days in an airtight container.