Preheat oven to 325°F.
Whisk flour, cocoa powder, baking powder, baking soda and salt in a small bowl. Set aside. Beat butter, sugar, brown sugar and eggs in a large mixing bowl 2 to 3 minutes until thick and well blended. Combine milk, corn syrup and vanilla in a small bowl. Add cocoa mixture to butter mixture, alternating with milk mixture, beating after each addition until smooth. Stir in chocolate chips.
Drop batter by heaping tablespoons 2 inches apart onto lightly greased baking sheets. Use moistened fingers to flatten batter slightly to a 1-1/2 inch diameter. Recipe should make 40 small "pies", which will be 20 whole Whoopie Pies when filled. (For larger Whoopie Pies, drop batter into 24 portions which will make 12 whole Whoopie Pies. Flatten larger portions to a 2-1/2 inch diameter.)
Bake 8 to 10 minutes (11 to 14 minutes for larger cakes) or until center is set and slightly firm to touch. Place pan on wire rack and let cakes cool completely.
For White Chocolate Filling:
Beat butter and powdered sugar in large mixing bowl until creamy. Beat in vanilla and melted white chocolate until very light and fluffy. Cover and chill until ready to use.
For Chocolate Glaze:
Melt chocolate chips, butter and corn syrup in a small bowl in microwave according to chocolate chips package directions. Stir until smooth.
To assemble Chocolate Whoopie Pies:
Spread filling on the bottoms of half of the pies; top with remaining pies. Spread glaze over assembled pies. Let stand until topping is set. (TIP: Refrigerate to set topping quickly.) Store in an air tight container.
Mint Variation: To make a Mint White Chocolate filling, add 1/2 teaspoon mint extract and a few drops of green food coloring.