In a medium bowl, mix together flour, sugar, baking powder, and salt until combined.
In another bowl, whisk together milk, eggs, and vanilla extract (if using).
Gradually add the dry ingredients to the wet ingredients while whisking until you have a smooth batter. Set aside.
Fill a medium saucepan 2-3 inches deep with vegetable oil. Heat it on the stove over medium-low heat. Use a candy thermometer in the oil to ensure it reaches 370-375°F (187-190°C).
Once the oil is hot, pour about ⅓ – ½ cup of batter into a liquid measuring cup. Slowly drizzle the batter into the hot oil in a thin stream, swirling it around to create patterns. Fry each side until golden brown, approximately 90 seconds per side. Remove and place on a paper-towel lined plate.
Dust the funnel cakes with powdered sugar and serve them warm.
Repeat the frying process with the remaining batter, ensuring the oil returns to the proper temperature between batches.