In a small bowl, whisk together the flour, baking powder, and salt.
In a medium bowl, stir together the melted butter and sugar until combined. Add the egg yolk, vanilla, and milk and whisk until smooth.
Pour the wet ingredients into the dry and stir gently until no streaks of flour remain, then fold in the blueberries with a spatula.
Spoon the batter into a buttered 10-ounce ramekin and set the ramekin on a small rimmed baking sheet.
Bake for 15 to 20 minutes, until the top is golden and a toothpick inserted in the center comes out with just a few moist crumbs.
Let the muffin cool in the ramekin for 10 minutes before enjoying.
Notes
Use a 10-ounce ramekin. Use a 10-ounce ramekin about 4.3 inches wide and 2.4 inches tall. A larger dish bakes faster and a smaller one takes longer.Measure the flour correctly. Spoon the flour into your measuring cup and level it off with a knife. For a single serving recipe with only a half cup of flour, an extra packed spoonful shifts the texture noticeably.Check that your baking powder is fresh. Baking powder is the only leavener in this muffin, so it does all the lifting. To test it, stir ¼ teaspoon into ½ cup of hot water; if it bubbles right away, it's still good.Don't overmix the batter. Stir just until no streaks of flour remain, then stop. Mixing past that point develops gluten and makes the crumb tough.Bake on a rimmed baking sheet. Set the ramekin on a small rimmed baking sheet before it goes in the oven. The sheet makes it easier to move in and out and catches any drips.Start checking doneness a few minutes early. Ovens vary in temperature, so the muffin may finish before the timer is up. Insert a toothpick in the center; just a few moist crumbs means it's done.