Preheat oven to 425°F and spray a standard doughnut pan with nonstick spray. Whisk together cake flour, sugar, baking powder, and salt in a large bowl.
Combine milk, eggs, melted butter, and vanilla paste in a medium bowl; add to flour mixture and fold in just until incorporated.
Spoon batter into a pastry bag fitted with a large round tip and fill each doughnut cup 3/4 full. Bake about 8 minutes, until the tops spring back when touched. Turn doughnuts out on a wire rack to cool completely. Repeat with remaining batter, allowing pan to cool completely and spraying with nonstick spray in between batches.
For the glaze, whisk together melted butter, powdered sugar, salt, vanilla paste, and milk until smooth. Set rack with cooled doughnuts over a piece of waxed paper. Dip doughnut tops into glaze. If desired, top with sprinkles and let set before serving.
NOTES: To make your own cake flour: place 2 tablespoons cornstarch in an empty 1-cup measure, then top up with flour. Sift the mixture several times to evenly distribute the cornstarch. This recipe was prepared for Rodelle by @tutti_dolci.
