Put the sugar, oil and vanilla extract in the bowl of a stand mixer and, using the paddle attachment, beat for 2-3 minutes until combined. Alternatively, beat well with a wooden spoon.
Add the egg and beat again for 2-3 minutes.
Slowly beat in the flour, cocoa, baking powder and salt. The mixture will look like raw cake mix at this stage but that’s ok. Cover with clingfilm and chill for at least 2 hours, but preferably overnight, to firm up.
When ready to bake, heat the oven to 180C, 160C fan oven, gas mark 4. Line a baking tray with non-stick baking parchment and put the icing sugar in a small bowl.
With fingers dusted with icing sugar, pinch off small pieces of dough and roll into walnut sized balls. Press the balls into flat discs and put a generous ½ tsp of conserve in the middle. Bring up the edges of the dough and pinch together to seal in the conserve. Roll again lightly into a ball and drop into the icing sugar. Roll around in the sugar until thoroughly coated and drop, seam side down, onto the lined tray.
Bake the cookies for 7-10 minutes or until they have risen and crinkled.Leave to cool then store in an airtight jar.
Make sure that there is plenty of icing sugar on the cookies before baking as some will melt into the dough, but the excess forms the powdered effect on the outside. Don’t worry if a little conserve is showing before baking.
The mixture will make 6-8 larger cookies, just bake for 3-4 minutes longer.
The cookies will keep in an airtight container for up to 4 days.