In stand mixer, combine sugars, butter and vanilla and mix on medium speed until light and fluffy, about 2 minutes. Beat in eggs one at a time until well blended.
Sift flours, cocoa and salt in medium bowl. Gradually add the flour mixture to butter mixture with mixer on low speed and blend until ingredients are fully combined. (no more than 1 minute)
Pour batter into prepared pan and bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.