Preheat oven to 180°C and line 2 trays with baking paper.
Cream the butter and demerara sugar for at least 5 minutes until the mixture becomes light and fluffy.
Add the egg and vanilla extract and beat well.
Add flour, baking powder, spices and salt and beat on low speed until just combined (don't overwork the mixture or it can become tough).
Stir through the rolled oats and chopped dark chocolate until evenly distributed.
Use an ice cream scoop to measure equal balls of cookie dough and place them onto the lined baking trays. I fit 6 per sheet.
Bake until cookies are golden around the edges, around 12-14 minutes.