Preheat oven to 400°F (200°C). Lightly grease a 10-ounce ramekin and set aside.
In a medium bowl, whisk together melted butter and sugar. Add the egg yolk and whisk until smooth. Whisk in lemon zest, vanilla, milk, and lemon juice.
In a small bowl, stir together flour, baking powder, and salt.
Add the dry ingredients to the wet ingredients and mix just until combined.
Gently fold in the blueberries.
Pour the batter into the prepared ramekin. Bake for 15 minutes, or until the top is golden and a toothpick inserted in the center comes out clean.
Remove from oven and let cool on a wire rack.
Make the Glaze: Mix powdered sugar with 1 teaspoon lemon juice and drizzle over the cooled muffin.
Video
Notes
Don’t Overmix: Mix just until combined. A few lumps are fine - overmixing can make the muffin tough.
Fold in Blueberries Gently: This helps keep them from bursting in the batter.
Grease the Ramekin: Coat your 10-ounce ramekin well to prevent sticking.
Use a Baking Sheet: Place the ramekin on a small baking sheet to catch any drips and make it easier to remove from the oven.