Preheat oven to 350ºF.

Prepare cake mix according to package directions.

Pour batter into a greased 10-inch bundt pan.

Bake cake according to package directions until a toothpick comes out clean.

Allow cake to cool for 10 minutes before running a knife around the edge and flipping it onto a wire rack.

Prepare the filling in a large mixing bowl by beating milk and pudding mix together for 2 minutes.

Gently fold in whipped topping and let stand for 5 minutes.

Using a knife, slice the bundt cake into two layers.

Place bottom layer of cake on a serving plate. Spread filling over top of it.

Place top layer of cake on top.

Prepare chocolate glaze by microwaving baking chocolate square and butter in a medium microwave-safe bowl in 30-second intervals until melted.  Stir often.

Add powdered sugar and milk.  Mix until smooth.

Drizzle chocolate glaze over top of cake, allowing it to drip down the sides.

Refrigerate the cake for 2-4 hours to allow the custard and glaze to set.

Slice and serve.