Preheat the oven to 350º F (175º C). Grease and flour large baking pan. Melt chocolate in small saucepan over low heat. Set aside. Combine the flour, cornstarch, salt, and baking soda in a medium bowl. Set aside. In mixer bowl, cream the butter and sugar on medium speed until light and fluffy, about 5 minutes. Gradually add the eggs, then sour cream and vanilla, scraping the bowl occasionally, until thoroughly mixed. On low speed, gradually add the flour mixture to the butter mixture, mixing until just combined. Pour two-thirds of the batter into the pan. Add chocolate to remaining batter, stirring until just combined. Place dollops of chocolate batter on top of yellow batter. Swirl together with a knife to marble (don't over do it!). Bake for 35-45 minutes, or until a toothpick inserted in center comes out clean. Cool completely in pan on wire rack.
In mixer bowl, beat butter and vanilla on medium speed until fluffy. Gradually add powdered sugar, beating until combined. Frost cake in the pan.