Preheat the oven to 375°F (190°C) and line a large baking sheet with parchment paper.
Add the butter, sugar, brown sugar, vanilla, salt, baking powder, and baking soda to a large mixing bowl. Using a standing or hand mixer, mix on high speed until light and fluffy.
Add the egg and mix again until fully incorporated.
Add the flour and cinnamon, then mix on low speed until just combined.
Mix in the milk on low speed.
Fold in the chocolate and mincemeat with a spatula, mixing gently until evenly incorporated.
Use a tablespoon to measure the cookie dough and drop spoonfuls onto the prepared baking sheet. Leave about 3cm (1 in.) space between to allow for spread.
Bake for 10 minutes, turning the tray at the halfway mark to ensure even baking.
Cool the cookies on the tray for 5 minutes before moving to a wire rack to cool fully.
Once cool, top with icing sugar and store in an airtight container for up to 2 weeks. These freeze well.