In a large mixing bowl, combine the flour, baking powder, salt, sugar, and ground cinnamon (if using).
In a separate bowl, whisk together the melted butter, warm milk, and vanilla extract.
Gradually pour the wet ingredients into the dry ingredients, mixing well to form a soft dough. Knead the dough for a few minutes until it's smooth and elastic.
Cover the dough with a clean kitchen towel and let it rest for about 30 minutes to allow it to relax.
Heat vegetable oil in a deep frying pan or pot to around 350°F (175°C).
While the oil is heating, divide the dough into small golf ball-sized portions. Roll each portion into a smooth ball.
On a lightly floured surface, roll out each dough ball into a thin round disc, about 6-7 inches in diameter. You can use a rolling pin or your hands.
Carefully slide the rolled-out dough into the hot oil, one or two at a time, depending on the size of your pan. Fry the buñuelos for about 2-3 minutes on each side, or until they are golden brown and puffed up.
Use a slotted spoon or tongs to remove the buñuelos from the oil and place them on a plate lined with paper towels to drain any excess oil.
While the buñuelos are still warm, roll them in granulated sugar to coat them evenly. You can also sprinkle more cinnamon and sugar on top if desired.
Serve the buñuelos warm and enjoy them as a delightful treat!