Combine water, milk, sugar, salt and yeast in mixing bowl; stir. Set aside for 3-5 minutes while yeast begins to foam. Add oil.
Punch down or remove excess air from the dough. Pinch off balls of dough about the size of a golf ball - or turn dough out on a lightly floured surface and pinch to divide into 8 balls. Oil hands, flatten each dough ball into a disk about the size of a DVD. Place 1-2 tablespoons filling in center of each disk, bring edges of dough up over the filling and pinch edges to seal.
Heat 1-3 tablespoons oil in griddle or skillet over medium to medium-low heat (use enough oil to coat the bottom of the griddle). Place filled dough balls, seam side down in hot oil, flatten hotteok with a hotteok pancake press, spatula, or flat bottom cereal bowl brushed with oil; fry 2-3 minutes or until golden brown. Turn pancakes, flatten again and fry 2-3 minutes longer.
Tips: Pour a few tablespoons oil in a small bowl; use oil to keep hands oiled. Dip a brush in the oil to recoat cooking griddle. Sometimes cooks will cover pan with a lid for 30 seconds to help cook the Hotteok and melt the sugar into syrup.