Heat oven to 350°F (175°C). Line a light-colored muffin pan with paper liners.
Sift together cake flour, baking powder, and fine sea salt.
Whisk together vegetable oil, egg, egg yolk, and vanilla extract until combined.
Beat butter and granulated sugar in the bowl of a stand mixer fitted with a paddle attachment on medium high speed until light and fluffy, about 3 minutes. Slowly add the wet ingredients in two batches, allowing the mixer to fully beat each addition.
Turn the mixer to low speed, and add in half of the dry ingredients followed by half of the buttermilk. Repeat this process, alternating the flour and buttermilk. Mix just until combined, scraping down the sides of the bowl as needed (be careful not to overmix at this stage). The batter will be slightly thick.
Evenly divide 3 tablespoons of batter into 16 paper liners. Bake for 18 - 22 minutes until the tops are springy to the touch. You can test that the cupcakes are cooked by inserting a toothpick into the center of the cupcake. If the toothpick comes out clean, the cupcakes are fully baked. Allow cupcakes to cool in the pan for 5 minutes, then transfer to a wire cooling rack until completely cooled.