Cookie dough: Melt the butter and then add it to a large bowl with the oil, sugar, egg, lemon zest and juice. Mix for 1-2 miuntes until the mixture thickens slightly.
Add the sieved flour, baking powder, salt and turmeric, then stir until combined, but do not over mix it.
Resting the dough: Chill the dough for 30 minutes or longer, even overnight if possible. (see notes)
Preheat the oven 180°C / 160°C fan / 350°F / Gas 4
Line two baking trays with parchment paper or silicone mats and set them aside.
Shaping: Pour 35g of granulated sugar into one bowl and 35g of icing sugar into another bowl. Scoop the dough into 18-20 balls, I use a 35mm/1 tbsp cookie scoop. Roll each cookie ball lightly in granulated sugar, then generously in the icing sugar. (Rolling the cookie in granulated sugar helps to prevent the icing sugar from being absorbed into the dough, giving us that lovely crinkle effect.)
Place the coated cookie balls equally on the prepared baking trays. I use a silicone mat (30 x 40cm) on my baking tray and this allows me to bake a maximum of 12 cookies at a time. I also always bake the trays separately to avoid creating steam during the baking.
Baking: Bake for 11-12 minutes and the edges have set. (See notes)
Allow the cookies to cool for 8-10 minutes on the baking tray, then transfer to a wire rack to cool.