Whisk flour, baking powder, dough sugar, and salt in a bowl. Add egg, melted cooled butter, ½ cup warm water, and vanilla. Mix into a soft dough. If dry flour remains, add water 1 teaspoon at a time, using only as much as needed.

Knead on a lightly floured surface for 6–8 minutes until smooth and pliable. Divide into 12 equal balls, cover, and rest for 30 minutes. Meanwhile, mix the coating sugar and ground cinnamon in a shallow dish.

If using, simmer chopped piloncillo, the syrup water, and cinnamon stick in a small saucepan for 8–10 minutes until dissolved and lightly syrupy. Remove the stick. Keep warm; the syrup thickens further as it cools.

Roll each dough ball on a lightly floured surface into a 7–8-inch round, thin enough to see a faint shadow through it. Brush off loose flour. Keep rounds in a single layer under a clean light towel while you roll the rest. If dough springs back, let it relax for 5 minutes and try again.

Pour oil to a depth of about 1 inch in a wide heavy pan, leaving ample space above it. Heat to 350°F, checking with a frying thermometer. Set a wire rack over a baking sheet beside the stove. Keep water away from the hot oil.

Carefully lower one round into the oil. Fry for 30–45 seconds on the first side, pressing gently with a spider if needed to keep it from folding. Turn with tongs and fry the second side for 20–40 seconds, until golden and crisp. Adjust heat to keep oil near 350°F. Lift out and drain on the rack. Repeat with remaining rounds.

While each buñuelo is still warm, sprinkle both sides with cinnamon sugar. Serve soon after frying, with optional syrup in a small jug alongside. Let diners add syrup to their own portions so the rest stay crisp.