Preheat oven to 350°F (180°C) and line a 12-cup muffin tin with paper liners.

In the bowl of a stand mixer (or in a large bowl with a handheld mixer) combine cake flour, sugar, salt, baking powder, and baking soda. Mix on low for a few seconds to combine.

Add butter, oil, and vanilla extract and blend until combined, scraping down the sides of the bowl once or twice.

Add the eggs and yolk, one at a time, and mix on low until fully combined.

Pour in the buttermilk and mix on low until thoroughly combined, but take care not to overmix.

Scoop batter evenly into the 12 prepared muffin cups and bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean. Let cool in the pan for 10 minutes before lifting onto a wire rack to cool completely.

Decorate the cupcakes with Vanilla Buttercream Frosting and sprinkles Store cupcakes in an airtight container at room temperature for up to 2 days. For longer storage, they can be frozen in an airtight container for up to 2 months. Defrost at room temperature for about 2 hours.