In a medium saucepan, add the Biscoff cookie butter, light brown sugar, cinnamon, nutmeg, and water.
Bring to a gentle simmer over medium heat, whisking to dissolve the sugar and melt the cookie butter.
Simmer for 10–15 minutes, stirring occasionally, until slightly thickened.
Remove from heat and stir in the vanilla bean paste or extract.
Strain through a fine mesh sieve into a clean jar or bottle. Let cool before sealing. Store in the fridge.