Combine flour, 1 cup melted butter, white sugar, milk, egg, and yeast in a large bowl. Knead until dough is smooth and elastic. Cover loosely with plastic wrap and let rise, about 45 minutes.
Heat treacle, brown sugar, 1/4 cup butter, and cinnamon in a small saucepan over medium heat until melted together, 3 to 5 minutes. Mix well until filling is smooth.
Preheat a shallow waffle iron according to manufacturer's instructions.
Knead dough briefly and divide it into balls about 1 1/2 inches in diameter, or as needed to fit your waffle iron.
Bake 1 ball of dough the preheated iron until golden brown and iron stops steaming, 30 seconds to 1 minute. Remove with a cake spatula. Press down with a round cutter to trim edges for a perfect circle. Carefully slice horizontally into 2 rounds while still hot. Spread 1 to 2 tablespoons of the filling on each round and sandwich together. Repeat with remaining dough.
Making stroopwafels does require a special shallow waffle iron, as regular waffle irons will produce waffles too thick for this purpose. Try finding a stroopwafel iron, if possible, or improvise with either a pizzelle iron or waffle ice cream cone iron.
Parchment can be used for easier cleanup/removal from the pan.
Per Serving: 559 calories;20.2 g fat;
92.5 g carbohydrates;
5.7 g protein;
67 mg cholesterol;
175 mg sodium.