Preheat oven to 350°F (175°C). Line a muffin tin with cupcake liners.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
In a separate bowl, cream together butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.
Fill cupcake liners about 2/3 full. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Let cool in the tin for a few minutes before transferring to a wire rack to cool completely.
Notes
Decorate with beach-themed sprinkles, frosting, and edible decorations.