Start heating the vegetable oil in a large, deep-sided pan on medium heat. Clip a candy thermometer to the side of the pan to monitor the temperature of the oil.
In a small bowl whisk together the all-purpose flour, granulated sugar, baking powder, and salt.
In a medium-sized bowl whisk up the egg, then add whole milk, water, and vanilla and whisk again.
Add the dry ingredients to the wet ingredients and mix well (Note: the batter should not be too thick).
Once the oil reaches 375F (190.5C), pour the batter into a kitchen funnel ½ cup at a time and quickly turn the funnel around in a circular motion to dispense the batter into the oil.
Fry for about 1 minute per side until golden. Once golden, remove the funnel cake from the oil and allow to rest on a paper towel to soak up any excess oil. TIP - You can use a piping or Ziploc bag and cut the corner 1.5 cm wide if you do not have a funnel.
Repeat the process until you've used up all the batter.
Notes
I recommend using vegetable oil for frying your funnel cakes, but other neutral oils such as canola oil or peanut oil will also work.
You can prepare the funnel cake batter a day in advance and store it in a large ziptop bag or container kept in the refrigerator until you are ready to fry them.
Though funnel cakes are best served fresh and warm, you can store them in the refrigerator in an air-tight container or a large ziptop bag for up to 3 days or at room temperature (in an air-tight container or bag) for 2 days. Reheat in the oven or air fryer so they aren't soggy.