In a saucepan, heat heavy cream or whipped topping over medium heat until almost boiling.

Remove from heat and add chocolate, stirring until melted and smooth. Let cool to room temperature.

Draw a six-inch circle on a piece of parchment paper. Turn the parchment upside-down and set on a plate or in a pan for support.

Spoon the chocolate mixture onto the circle, about one and a half inches wide, to form a ring. Freeze while preparing the cake batter.

Preheat oven to 350 degrees Fahrenheit. Grease and flour a 10- or 12-cup Bundt pan.

Melt chocolate in a double boiler or in the microwave. Set aside.

In a large bowl, sift together flour, cocoa powder, baking powder, baking soda, and salt. Set aside.

In an electric mixer, beat butter or margarine until light and fluffy, about one minute.

With the mixer running, gradually add the sugar.

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

Add one-third of the flour mixture at a time, alternating with one-third of the buttermilk and mixing well after each addition. Continue until all of the flour mixture and buttermilk have been added.

Add melted chocolate and mix until no streaks of chocolate are visible.

Pour two cups of batter into the prepared pan.

Remove fudge ring from the freezer and transfer it onto the cake batter.

Pour remaining batter over the ring.

Bake 45 minutes, until cake begins to pull away from the sides of the pan and the surface of the cake is cracked.

With the cake still in the pan, place on a cooling rack for 20 minutes. This allows the filling to set and makes it easier to remove the cake.

Invert onto a serving platter. Serve warm with ice cream or enjoy at room temperature.