Preheat your oven to 375°F (190°C). Grease and flour two 12-cup cupcake pans or line them with cupcake liners.
In a large mixing bowl or the bowl of a stand mixer, whisk together the cake flour, sugar, baking soda, baking powder, and salt until well combined.
Add the softened butter to the dry ingredients. Mix on medium speed until the mixture resembles fine crumbs.
In a separate bowl, whisk together the buttermilk, vegetable oil, eggs, egg yolk, and vanilla bean paste until smooth.
Gradually add half of the wet ingredients to the dry mixture. Beat on medium speed until the batter is lighter in color and smooth (about 1-2 minutes).
Add the remaining wet ingredients and mix until fully incorporated, making sure to scrape down the sides of the bowl.
Divide the batter evenly between the prepared cupcake liners, filling each about ⅔ full.
Bake in the preheated oven at 375°F for 5 minutes.
After 5 minutes, reduce the oven temperature to 350°F (175°C) and bake for an additional 10-12 minutes, or until a toothpick inserted into the center of a cupcake comes out clean.
Remove the cupcakes from the oven and let them cool in the pans for 1-2 minutes. Then transfer the cupcakes to a wire rack to cool completely before frosting.