Combine dry ingredients and cut in butter until well mixed. Add egg yolk and vanilla and blend thoroughly.
Dust fingers with flour and pat dough onto bottom and about 1" up on sides of 10" cheesecake pan. (I use 10" Kaiser springform pan with non-stick finish.)
Filling: Beat cream cheese in large mixer bowl until smooth and creamy. Combine dry ingredients; stir into cream cheese mixture. Add eggs and beat just until combined (do not overbeat). Stir in the chocolate, sour cream and vanilla, mixing until combined. Pour into crust-lined pan.
Bake in 325 degrees for 60-70 minutes or until center appears set or cracks. Remove from oven and cool 15 minutes. Run spatula around edge of pan to loosen cheesecake. Cool 30 minutes more. Remove sides of pan, cover and chill at least 2 hours.
Top: Melt semisweet chocolate and butter. Drizzle in lattice design on top cheesecake. Chill till trim sets. Makes 12 servings. Store in refrigerator.
York-style or plain cheesecake, I add 1 tsp. Grated orange or lemon zest. Also, a chocolate crust can be used for the above filling: 1 1/2 C. finely crushed chocolate wafers mixed with 6 T. melted butter. Mix and pat into pan.