Start by heat-treating the flour to kill any bacteria. This can be done in the microwave, in a frying pan or in the oven. If using a microwave, tip the flour onto a plate and microwave on medium–full power for 90 seconds, stirring the flour every 15 seconds. For the frying pan method, tip the flour into a heavy-based frying pan and place over a medium heat, stirring almost constantly for about 4 minutes until the flour is hot. To heat-treat flour in the oven, preheat the oven to 170C/150C Fan/Gas 3½, tip the flour onto a baking tray in an even layer and heat for 5 minutes until the flour is hot, stirring halfway through. The flour must be cooled to room temperature before mixing the cookie dough.