In a large bowl, cream together butter and both sugars until fluffy using an electric mixer.
In a seaparate small bowl mix together cornflour, water, and molasses.
Add the cornflour mix to whipped butter and whisk well.
Sift together all the remaining dry ingredients; flour, baking soda, baking powder, salt, ginger, cinnamon, and mixed spice.
Add the flour mixture to the whipped butter mixture. Fold together with a spoon or soft spatula until just combined.
Cover the dough and refrigerate for 30 minutes.
In the meantime preheat the oven to 392°F / 200°C (fan oven setting) so it's ready for baking.
If you're not using a fan oven, preheat to 428°F / 220°C.
Remove the dough from the fridge and scoop out tablespoon-size balls.
Lightly roll the balls between your palms until smooth and perfectly round. Place them on a lined baking tray. Make sure to leave enough space for the cookies to expand.
Repeat until you have 9 cookies (or until there is no dough left).
Use your fingers to flatten each ball until it is about ¾ - 1 cm thick (0,4 inches thick).
Bake for 7-8 minutes until crisp around the edges but soft in the middle. Leave to cool for 10-15 minutes.