Syrup holds together the two baked layers of a stroopwafel.
A stroopwafel is made from two thin, round wafers of sweet dough. The warm baked discs are separated or trimmed, then joined with a filling traditionally based on syrup, brown sugar, butter, and cinnamon. As the filling cools, it thickens and binds the halves together.
The Dutch name explains the defining ingredient: stroop means syrup, while wafel means waffle. Many modern commercial versions describe the filling as caramel, and recipes can vary, but syrup remains the canonical answer to this question. Marzipan, custard, and jam are fillings associated with other pastries, not the standard stroopwafel.
The treat originated in Gouda, in South Holland. Culinary tradition places its beginnings around the late 18th or early 19th century, possibly as a way for bakers to sweeten leftover crumbs. Stroopwafels are commonly placed over a hot drink so steam softens the syrup center.