1) Preheat the oven to 350°F. Grease and flour two 9" round cake pans, three 8" round pans, or a 9" x 13" sheet cake pan.
2) In a large mixing bowl, cream together the butter, sugars and salt till fluffy and light, at least 5 minutes. Stop to scrape the sides and bottom of the bowl at least once during that time. In a separate bowl, whisk together the flours, cocoa and baking soda.
3) Add the eggs to the butter mixture one at a time, beating well (at least a full minute) after each addition, and stopping to scrape the sides and bottom of the bowl before adding the next egg. Mix in the vanilla. Add half the flour/cocoa mixture, and mix until evenly combined. Add the yogurt, and mix again. Add the remaining flour, mix, and stop to scrape the sides and bottom of the bowl once more, to make sure the batter is thoroughly combined. Divide the batter evenly between the pans.
4) Bake the cakes for 30 to 35 minutes (5 to 7 minutes longer for the sheet cake), until a cake tester inserted into the center comes out clean, and the sides of the cake begin to pull away from the pan. Remove the cakes from the oven, cool them for 5 to 10 minutes, then remove them from the pan. (The sheet cake may be served right from the pan.)