Preheat oven to 350°F (175°C).
Grease a 6-inch cake pan and line the bottom with parchment paper.
In a bowl, whisk together flour, sugar, baking powder, and salt.
In another bowl, mix melted butter, egg, milk, and vanilla extract.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined (do not overmix).
Pour batter into the prepared pan and smooth the top.
Bake for 20-25 minutes or until a toothpick inserted comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack.
Serve plain or with frosting, berries, or whipped cream.
