Preheat your oven to 350°F (175°C).
Line a muffin tin with cupcake liners.
In a mixing bowl, whisk together the all-purpose flour, baking powder, and salt.
In another bowl, combine the melted butter and granulated sugar. Beat them together until light and fluffy.
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract or vanilla bean paste.
Gradually mix in the buttermilk until the batter is smooth.
Slowly add the dry ingredients to the wet mixture, stirring gently until just combined.
Gently fold in the rainbow sprinkles.
Scoop the batter into the prepared muffin tin, filling each liner about two-thirds full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick comes out clean.
Let the cupcakes cool in the tin for 5 minutes, then transfer to a cooling rack.