Cream butter until soft; add sugar gradually and cream until fluffy (about 5 minutes).
Sift flour, baking powder and salt; add coriander.
Combine milk and sour cream.
Add the dry mixture to the butter-sugar mixture alternately with the milk, beating after each addition.
Pinch off pieces about the size of a hazlenut. Roll in balls, place about 2 1/2 inches apart on greased cookie sheets. Flatten in circles (using the bottom of a glass dipped on flour). Bake about 12 minutes or until the edges begin to brown. Cookies may be sprinkled with cinnamon, sugar, chopped nuts, or colored sugar before baking.