Prep your tools and oven. Line a 12-cup muffin tin with cupcake liners. Preheat the oven to 350°F (175°C). Make sure your butter, eggs, and milk are at room temperature for the best texture.

Whisk dry ingredients. In a medium bowl, whisk together flour, baking powder, baking soda, and salt. This helps keep the batter light and lumps to a minimum.

Cream butter and sugar. In a large bowl, beat the softened butter and sugar with a hand or stand mixer on medium speed until pale and fluffy, about 2–3 minutes. Proper creaming traps air, giving the cupcakes lift.

Add oil, eggs, and vanilla. Beat in the oil until combined. Add eggs one at a time, mixing well after each. Stir in the vanilla. Scrape down the bowl so everything blends evenly.

Alternate dry ingredients and milk. On low speed, add half the dry mixture, then the milk and sour cream, and finally the remaining dry ingredients. Mix just until combined. Do not overmix or the cupcakes can turn dense.

Fill the liners. Divide the batter evenly among the liners, filling each about two-thirds full. A cookie scoop helps keep them uniform.

Bake. Bake for 16–20 minutes, or until the tops spring back and a toothpick comes out clean or with a few moist crumbs. Rotate the pan once if your oven has hot spots.

Cool completely. Let cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool fully before frosting. Warm cupcakes will melt the buttercream.

Make the buttercream. Beat the softened butter on medium-high until smooth and slightly pale, about 2 minutes. Add half the powdered sugar and a pinch of salt; mix on low until combined. Add vanilla and 2 tablespoons cream, then the remaining sugar. Beat on medium-high until light and fluffy, 2–3 minutes. Adjust with more cream for a softer frosting or more sugar for a stiffer one. Taste and add a tiny bit more salt if needed to balance sweetness.

Frost and decorate. Use a knife or piping bag with your favorite tip to frost. Add sprinkles if you like. Serve at room temperature for the best flavor and texture.