In a medium saucepan, stir together sugars, cornstarch and spices.
Add in apple cider and lemon juice, then stir until well-combined.
Bring mixture to a boil over medium heat. Reduce heat slightly to medium low and allow to simmer for approximately 20-30 minutes (until thickened slightly), stirring occasionally.
Remove from heat and stir in butter until melted throughout.
Allow to cool slightly and transfer to bottles.
Store in the refrigerator.
Notes
* for thicker syrup, allow the mixture to simmer longer (or add in a little more cornstarch). * the melted butter in the syrup with solidify some in the fridge. So, to serve, just remove the cap from the bottle and reheat the syrup in the microwave for about 20 seconds to a minute to melt that butter again and warm the syrup!