1. Sprinkle cavity of bird with salt. Place in a large kettle.
2. Add stock to kettle. Cover. Simmer until just tender Allow to cool.
3. Meanwhile, cream egg yolks and butter; add flour and blend thoroughly. Stir in sour cream. Season with 1 teaspoon salt and pepper. Beat at high speed until stiff. Cook until thickened in top of a double boiler, stirring constantly to keep from curdling or sticking (handle like hollandaise sauce). Cool.
4. Make cuts in capon as for carving, but without cutting through. Place in a shallow baking pan. Fill cuts with sauce, then spread remainder over the whole surface of the bird.
5. Bake at 425°F about 20 minutes, or until sauce is browned.
6. Meanwhile, boil liquid in which chicken was cooked until it is reduced to 1 cup of stock.