Preheat the oven to 350°F (180°C). Line a 12-cup muffin tin with cupcake liners.
In a medium bowl, whisk together the flour, sugar, baking powder, and salt.
In a large bowl, using an electric mixer, beat the butter until creamy. Add the eggs one at a time, beating well after each addition.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Beat just until combined.
Stir in the vanilla extract.
Divide the batter evenly among the muffin cups.
Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean.
Allow the cupcakes to cool completely in the pan.
For the frosting, beat the butter until creamy. Gradually add the powdered sugar, beating until smooth and creamy.
Add the milk and vanilla extract, beating until combined.
Divide the frosting among the cooled cupcakes and decorate with gel food coloring, sprinkles, and candy eyeballs.