Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a large bowl, whisk together the flour, cornstarch, cocoa powder, cinnamon, and salt until thoroughly blended, ensuring that no cocoa powder lumps remain.
In the bowl of a stand mixer, using the paddle attachment, cream butter and the Redpath® Demerara Style Sugar and Redpath® Granulated Sugar until fluffy and pale in colour; about 3 minutes. Mix in the vanilla extract.
Add the dry ingredients into the bowl. On low speed, stir until just combined. Add in the chopped chocolate, mix on low speed until chocolate is evenly mixed throughout the dough.
Form dough into a flat disc and wrap well in plastic wrap. Place into the fridge to chill and rest; 30 minutes.
Place dough between 2 sheets of parchment paper. Roll to a ¼-inch thickness. With desired cookie cutters, cut out shortbread. Place onto the prepared baking sheet and place into the freezer to chill; at least 15 minutes.
Place chilled cookies into the preheated oven and bake for 7 to 8 minutes, rotating halfway through.
Remove from oven and allow to cool completely on the baking sheet before removing.