Heat vegetable oil in a deep fryer or heavy-bottomed pot to 375F (190C).

In a mixing bowl, whisk together the flour, sugar, baking powder, and salt.

In a separate small bowl, whisk the egg and milk until combined.

Whisk the wet ingredients into the dry ingredients until a smooth batter is formed.

Dip marshmallows into the batter individually using a fork or skewer to ensure even coating.

Gently place coated marshmallows into the hot oil, frying in small batches to avoid overcrowding.

Fry for 45 to 60 seconds or until the batter is golden brown and crispy.

Remove with a slotted spoon and drain briefly on paper towels.

Dust with powdered sugar and serve immediately while the interior is molten.