Grease a 9 inch square baking pan. You can line it with parchment if you'd like, but it's not absolutely necessary.
Make the crumb mixture first. Mix together flour, sugar and both zests, then use a grater to grate in the cold butter, tossing occasionally to mix. You can also do this with a pastry cutter, two forks or a food processor, but the easiest way is just to grate it in there. Cover and keep chilled while you make the cake batter.
Mix together the cake flour, baking soda, salt, and lemon zest then set aside.
In a mixing bowl using an electric mixer, beat the ½ cup of softened butter with the 1 ½ cups of sugar, scraping sides of bowl often. Add egg yolks, one at a time, beating for 30 seconds after each egg is added.
Add the flour mixture alternately with the yogurt, stirring by hand or on lowest speed until blended.
Beat the egg whites until soft peaks for, then fold ⅓ at a time into the cake batter. Batter will be thick.
Scrape the thick batter into the pan and spread evenly. Drop dollops of lemon mixture over the top and swirl with a knife. Sprinkle crumb mixture over the top. It will seem like too much, but it's not.
Bake for 45 to 50 minutes at 350 degrees F. Let cool.