Beat butter and vanilla extract, then add sugar gradually, beating thoroughly.
Add eggs, one at a time, beating 1 minute after each addition.
Combine flour, baking powder, and salt.
Add half of flour to creamed mixture and mix with a wooden spoon.
Add remaining flour; mix well.
Divide dough into four portions.
Roll each portion into a long roll about 1 1/2 inches in diameter on a floured pastry cloth.
Place rolls 3 inches apart on a well greased and floured cookie sheet.
Flatten dough slightly with hands.
Brush tops with egg yolk.
Bake at 400 degree F about 20 to 25 minutes, or until golden and firm to the touch.
Remove from oven.
Cover with a towel; let stand 2 hours.
Cut rolls into 1/2 inch slices.
Place on ungreased cookie sheet, cut sides down.
Toast in a 400Â°F oven 15 to 20 minutes, turning several times until delicately brown.