Preheat oven to 350°F. Grease and flour two 9-inch round cake pans; set aside.
Combine dry ingredients together; set aside. Combine sour cream and milk; set aside.
Beat butter on medium-low speed of electric mixer until creamy, about 2 minutes. Add sugar and vanilla bean seeds; mix on medium speed until the mixture is fluffy and pale in colour, about 2 to 4 minutes. Scrape sides of bowl.
Add eggs and eggs yolks, one at a time, beating on low speed until blended. Add vanilla extract; beat on low speed until blended.
Add in half of the flour mixture. Once this is incorporated, pour in the milk mixture while the mixer is running on low speed. Once the milk is absorbed, add in the remaining flour mixture. After the last streaks of flour are incorporated, mix the batter on medium-low speed for about 20 seconds until smooth.
Pour batter into prepared cake pans. Bake for 35 to 40 minutes or until wooden pick inserted in centre comes out clean.
Cool cakes on a wire rack for about 15 minutes or until pans are cool enough to handle; remove cakes from pans. Continue to cool on wire rack until cakes are at room temperature.
Wrap each layer in a double layer of plastic wrap; chill in refrigerator for at least 2 hours or up to 2 days before cutting and frosting. (Chilling makes the cakes easy to cut.)
When ready to frost, cut each layer horizontally in half; frost with Whipped Vanilla Buttercream Icing.
Recipe Tip: If your vanilla bean is not soft and flexible, soak in hot tap water until pliable. Place bean on a cutting board and slice lengthwise, using a sharp knife or razor. Open bean and scrape seeds out using the tip of a knife.
Recipe courtesy of Tessa Huff, Pastry Chef & Blogger @ StyleSweetCA https://www.stylesweetca.com/