Cream the butter, sugar and vanilla extract together in an electric mixer until very light and fluffy. About 5 minutes should do it, scraping down the sides as needed.
Add in the flour, cornflour, baking powder and salt. Gently mix until the dough just comes together.
Fold in the crystallised violets.
Tip the dough out onto a lightly floured surface, and bring it together and form it into a log which is about 3 inches (7.5cm) thick. Wrap the dough log in clingfilm/plastic wrap and place it in the fridge for at least an hour. Or up to 3 days.
Once you are ready to bake your cookies preheat your oven to 180c/350f/Gas mark 3 and line a large baking sheet with baking parchment or a large silicone mat.
Slice your cookies to about 1 inch (2.5cm) thick and place them on your baking sheet. These cookies will not spread very much.
Bake them for 18-20 minutes until they are golden brown. Remove them from the oven and transfer them to a wire rack to allow them to fully cool down.
These cookies will keep for 5 days in an airtight container at room temperature.