Preheat oven to 375°F. Grease a cookie sheet or line with parchment paper or a silpat mat; set aside.
Chop 4 ounces of the baking bar; set aside. In a saucepan melt the remaining 4 ounces of baking bar over low heat, stir constantly. Let cool.
Use an electric mixer to beat the butter about 30 seconds. Add the sugar, baking soda, and salt. Beat until combined. Beat in the eggs and melted baking bar until combined. Beat in as much flour as you can with the mixer. Stir in any remaining flour. Stir in the 4 ounces of chopped white baking bar.
Drop the dough from rounded teaspoons 2 inches apart onto the prepared cookie sheet. Bake in preheated oven 7 to 9 minutes or until cookies are lightly browned around edges. Cool on cookie sheet for 1 minute. Transfer cookies to wire racks; cool.
Just before serving, in a saucepan melt the jam over low heat. Spoon about 1/2 teaspoon jam atop each cookie. In a heavy saucepan melt 3 ounces white baking bar and shortening over low heat; stir constantly. Drizzle cookies with melted mixture. If necessary, chill cookies for 15 minutes or until baking bar mixture is firm.
Place in layers separated by waxed paper in an airtight container; cover. Store in refrigerator up to 3 days or freeze plain cookies up to 3 months. Thaw, then top cookies with jam and drizzle with melted baking bar mixture.