Substitute for demerara sugar in cookie recipe
Demerara sugar has a syrupy flavor which is between light muscovado sugar and granulated sugar, I would substitute a 50/50 mix of golden granulated and light muscovado sugar. As you have some demerara you could do a small amount of 50/50 mix and taste test, adjusting to get the right balance. 

You could also put in straight light muscovado if you wanted a richer result, or golden granulated if you like it slightly lighter, cookies are generally pretty flexible on this.