That’s a good question! Have you made a similar cake with added cinnamon? If so, I’d add the same amount of speculaas spice. If not, I’d try a couple teaspoons or even a tablespoon (or two), depending on how strong you want the flavor to be. Whenever I add speculaas spice to something I’m baking I add a teaspoon at a time and taste it, then add more, so if at all possible, I’d add a couple teaspoons and then bake a cupcake-sized portion to test the spice level. Let me know how it turns out!! (Now I really want to go make a speculaas spice cake with cream cheese icing.) 🙂