Preheat the oven to 350 degrees F. Butter a 1-quart soufflé dish. (You can use smaller dishes or ramekins, if you want individual servings).
In the bowl of a stand mixer, beat the butter at medium speed until light and fluffy. Add the sugar and lemon 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 and beat until smooth; repeat with remaining lemon juice, flour and sour cream.
Beat the egg whites at medium speed until foamy. Increase speed to medium-high, add the salt, and beat to stiff 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 it to the prepared soufflé dish or dishes. Place the dish or dishes in a larger pan and carefully pour boiling water around it to a depth of 1 inch.
Bake for 50-60 minutes or until slightly golden brown and the center is just set. The cake should spring back when gently touched. Do not open the oven door while the cake is baking. Remove from the water bath immediately and cool on a rack for 10-15 minutes.
Lightly dust with powdered sugar. Serve warm.
Notes-if you can't find Meyer lemons you can use regular lemons. Just use 1/4 cup of lemon juice. And if you use smaller dishes or ramekins decrease baking time to 40-45 minutes, or until golden brown. Just watch the cakes to be safe.