Preheat oven to 350 degrees F (175 degrees C).

Combine chocolate cookie crumbs, chopped pecans, and melted butter for the crust.

Press the crust mixture into a 9-inch springform pan.

Bake the crust for 7-10 minutes.

In a large bowl, combine softened cream cheese, sugar, cocoa powder, and cornstarch for the filling.

Beat with an electric mixer until smooth.

Add eggs and egg yolk, one at a time, beating well after each addition.

Beat in Irish cream liqueur and vanilla.

Pour the filling into the baked crust.

Bake at 350 degrees F (175 degrees C) for 15 minutes.

Reduce the oven temperature to 200 degrees F (95 degrees C) and bake for 1 hour and 10 minutes.

Remove the cake from the oven and run a knife around the inside edge of the pan to loosen it.

Turn the oven off and return the cake to the oven for an additional 30 minutes to cool slowly.

Let the cake cool completely at room temperature, then chill overnight in the refrigerator.

Remove the sides of the springform pan.

For the topping, melt the Hershey bar in the microwave for about 30-40 seconds, or until melted.

Stir in the Irish cream until smooth.

Drizzle the topping over the chilled cake.

Slice and serve.