Pre-heat the oven to 350F (180C). Grease and flour an 8 inch (20cm) cake pan or bundt pan.
In a large bowl whisk together the flours and sugars add the softened butter and combine until coarse crumbs appear, remove 1/2 cup and set aside.
Into the large bowl (with the crumbs) add the yogurt, eggs, salt, baking soda and vanilla, beat the ingredients together on low speed for 1 minute then raise the speed to medium and beat again for 2 minutes. The batter will be thick.
Spoon the batter into the prepared cake pan, smooth the top then sprinkle the crumbs (that were set aside) on the top. Bake for 40-45 minutes, test for doneness with a toothpick. Let cool completely before serving.
For room temperature ingredients, remove from the fridge 45-60 minutes before using.To make cake flour, measure 1 cup of all purpose flour and remove 2 tablespoons replace with 2 tablespoons of cornstarch, sift together before using.