In a large bowl, using an electric/stand mixer (or hand whisk, just double/triple the times) combine the butter and both sugars on medium speed for several minutes, until fully combined. Add the egg and vanilla and mix again.
Add the flour, oats, baking powder and cinnamon, and fold in using a rubber spatula or wooden spoon, until just combined. Add the raisins and combine. Let chill in the fridge for at least 30 minutes.
Preheat oven to 200C/180C Fan/Gas Mark 6. Line 2-3 large baking trays.
Scoop large balls of cookie dough – about 1 tbsp in size, so you have 12 of similar size. Place on the trays, with several inches in between (max 3 per tray).
Bake for about 12 minutes, until golden brown. They may look a little underbaked in the centre but they keep cooking from the residual heat, and this gives them their perfect soft texture. Repeat for all cookies and leave to cool on a wire rack before serving.