Melt the butter, heat the oven to 350°F (175°C), and line a muffin tin with paper liners.
6 tablespoons (85 grams) unsalted butter
In a large mixing bowl, whisk together the 2 cups of flour, sugar, baking powder, baking soda, salt, and jimmie-type sprinkles.
2 cups (250 grams) all-purpose flour, ½ cup (100 grams) granulated sugar, 2 tablespoons baking powder, ¼ teaspoon salt, ¼ teaspoon baking soda, ⅓ cup (64 grams) rainbow sprinkles
In a medium bowl or large glass measuring cup, combine the buttermilk, eggs, and vanilla extract and whisk until smooth.
1 cup (240 milliliters) buttermilk, 2 eggs, 1 tablespoon vanilla extract
Pour the wet ingredients and the melted butter into the dry ingredients and stir just until all visible flour has been stirred in. Do not over-mix.
6 tablespoons (85 grams) unsalted butter
Use a large cookie scoop or spoon to divide the batter evenly among the muffin cups, filling each about ¾ full.
Give the muffin pan a quick thwack on the countertop to spread out the batter, then sprinkle a teaspoon of nonpareils over the top of each one. You can give the pan a bit of a gentle shake to get any loose ones to adhere to the batter.
¼ cup (48 grams) rainbow nonpareils
Bake for 16 to 18 minutes or until a toothpick inserted into the center of a muffin comes out with no more than a few crumbs.
Allow the muffins to cool in the muffin pan for 5 to 10 minutes, then transfer them to a wire rack to cool completely.