Add the milk, active sourdough starter, egg, mostly melted unsalted butter, granulated sugar, ground cinnamon, salt, all-purpose flour, and vanilla extract to the mixing bowl.

Mix the waffle ingredients until a shaggy dough forms.

Transfer the dough to a clean work surface. Knead it for about 2 minutes until it forms a smooth, cohesive ball.

Divide the dough into 10 equal balls, about 43 g each.

Place the dough balls on the baking sheet. Cover them with lightly oiled plastic wrap or a damp towel.

Let the sourdough dough balls rise for 330 minutes, or let dry-yeast dough rise for 60 minutes. They should look slightly puffed rather than doubled.

About 20 minutes before cooking the waffles, add the Dutch stroop syrup, brown sugar, unsalted butter for filling, ground cinnamon for filling, and salt for filling to the saucepan.

Heat the filling over medium heat, stirring until the butter and sugar melt and the mixture just begins to boil.

Remove the stroop filling from the heat. Let it cool to about room temperature so it thickens slightly.

Preheat the stroopwafel iron fully. A krumkake iron or ice-cream-cone iron may be used instead.

Place 1 dough ball in the hot iron. Close the iron and cook until the waffle is evenly golden brown and crisp.

Remove 1 hot waffle from the iron. Carefully split it horizontally into two thin rounds with the sharp knife while it is still warm.

Spread about 1 1/2 tablespoons of stroop filling over the cut side of the bottom waffle round. Keep the filling slightly away from the edge.

Place the top waffle round over the filling. Gently press the layers together to seal.

Repeat the cooking, splitting, filling, and sealing process with the remaining dough balls. Let the finished stroopwafels cool until the filling sets, or serve warm.