Preheat the oven to 350°F (175°C). Lightly grease a mini muffin pan or line with mini paper liners.
In a large bowl, whisk together the all-purpose flour, granulated sugar, salt, and baking powder until well combined.
In a separate medium bowl, whisk the melted butter, milk, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together with a spatula until just combined. Do not overmix; some lumps are okay.
Spoon the batter into the prepared mini muffin pan, filling each cup about 3/4 full.
Bake for 14 to 16 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the pan for 5 minutes.
Transfer the mini muffins to a wire rack to cool completely before serving.