Preheat the oven to 350 degrees F. Spray or butter a 1 quart souffle dish (or a set of ramekins or a square baking dish that will fit into a larger baking dish). Also, have a larger baking pan ready that will accommodate your souffle dish or ramekins.
Beat the butter at medium speed until light. Add the sugar and zest and beat until combined. Add the egg yolks, one at a time, beating well after each addition. Reduce speed to low and add half the lemon juice, half the flour and half the sour cream/yogurt and beat until smooth; repeat with remaining lemon juice, flour and sour cream/yogurt. Pour this mixture into a separate bowl.
Next, beat the egg whites at medium speed until foamy. Increase speed to medium-high, add the salt, and beat to stiff peaks. Meanwhile, bring a few cups of water to a boil. Once egg whites have stiff pearly peaks, add one-quarter of the whites to the lemon mixture and gently fold in. Continue to fold in whites one-quarter at a time. Transfer the mixture to the prepared souffle dish or ramekins. Place in the larger pan and carefully pour boiling water around it to a depth of 1 inch.
Bake for 50 minutes to an hour, until the top is golden brown, the center is just set, and the top springs back when lightly touched. Remove from the water bath and cool for 10-15 minutes.
Lightly dust with powdered sugar and serve warm, scooping up some of the pudding at the bottom of the dish along with the cake.
*You'll be tempted to add something to this, a berry sauce, or some ice cream, but don't... it is SO good just by itself I promise!!