First, brown the butter. To do this, melt butter in medium sized sauce pan over medium heat. Stir constantly until butter turns amber in color and has a nutty smell, about 10 minutes. Transfer butter to a heat safe bowl and allow to re-harden for at least an hour. You can also put in the fridge and use again the next day.
When ready to make the cake, preheat oven to 350 degrees and line 3, 8" baking pans with parchment paper. Grease and flour sides (I use baking PAM for this).
Beat together butter, sugar and light brown sugar on medium high speed until lightened and fluffy, about 3 minutes.
With mixer on low, slowly beat in eggs, egg yolks, grapeseed oil, and vanilla.
In medium sized bowl, whisk together flour, corn starch, baking powder, kosher salt, and cinnamon.
Slowly beat about 1/2 cup flour mixture into sugar/butter mixture. Then, beat in about 1/3 of the buttermilk. Alternate beating in rest of flour and buttermilk, ending with the flour.
Spread batter evenly between the three prepared pans.
Bake for 23-25 minutes, or until tops are golden and toothpick in center of cake comes out with just a few crumbs.
Let cool completely before assembling cake.