Prep your pans and oven. Heat oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment. Lightly flour the sides and tap out the excess.

Whisk the dry mix. In a bowl, whisk flour, cornstarch, baking powder, baking soda, and salt. Set aside. This prevents clumps and ensures even rise.

Cream butter and sugar. In a large bowl or stand mixer, beat butter and sugar on medium-high until pale and fluffy, about 3 minutes. This step builds structure and air.

Add oil and vanilla. Mix in the oil, vanilla extract, and vanilla bean seeds or paste. Beat until glossy and well combined.

Add eggs and yolk. Beat in eggs one at a time, then the extra yolk. Scrape the bowl after each addition. Stop when smooth—don’t overmix.

Combine wet dairy. In a measuring cup, whisk milk and sour cream until smooth.

Alternate dry and wet. Add the dry ingredients in three parts, alternating with the milk mixture in two parts (dry-wet-dry-wet-dry). Mix on low just until combined. A few tiny lumps are okay.

Portion and bake. Divide batter evenly between pans. Smooth tops and tap pans on the counter to release air bubbles. Bake 25–30 minutes, until a toothpick comes out clean or with a few moist crumbs.

Cool completely. Let cakes cool in pans 10 minutes, then turn out onto a rack. Peel parchment and cool fully before frosting.

Make the frosting. Beat softened butter until creamy. Add powdered sugar, 1 cup at a time, mixing on low then medium. Add cream, vanilla, and a pinch of salt. Beat 2–3 minutes until fluffy. Adjust with more cream or sugar for spreadable consistency.

Assemble. Level cake tops if domed. Place one layer on a stand, spread frosting, then add the second layer. Crumb coat the whole cake with a thin layer of frosting. Chill 15 minutes, then apply a final coat. Add swirls or sprinkles if you like.