Preheat oven to 350 F.
Lightly coat the Bundt pan and tunnel insert with cooking spray and dust with flour; set aside.
In a small bowl, sift together the flour, baking powder, baking soda and salt; set aside.
In a large bowl, using an electric mixer set on medium-high speed, cream the butter and sugars until light and fluffy. Add the eggs, one at a time, beating well after each addition. With the mixer running, drizzle in the cooled chocolate, mixing just until incorporated.
Reduce the mixer speed to low. Add the sour cream and the flour mixture, alternating in halves, beginning with the sour cream and ending with the flour. Mix just until blended.
Spoon the batter into the prepared Bundt pan and spread it around evenly with a rubber spatula. Place the tunnel insert on top of the pan. Bake for 45 to 55 minutes, or until a toothpick inserted into the center comes out clean. Let the cake rest, set on a rack, for 5 minutes.
Slowly remove the tunnel insert. (Note: If the cake has risen over the pan rim, trim off the excess before removing the insert.) Let cool for 10 minutes and then place a cooling rack on top of the cake, invert the cake, and remove the pan. Cool completely.
To make the filling: In a medium saucepan over moderate heat, warm the cream until bubbles form on the edges. Add the chocolate and stir until the chocolate melts and incorporates into the cream. Remove from the heat, add the powdered milk and whisk until completely dissolved. Strain the chocolate cream into a medium bowl. Chill until cool but not set, about 15 minutes. Beat the cream until stiff.
To fill the cake, invert the cooled cake to reveal the tunnel. Fill the tunnel with the chocolate malt filling, spreading it evenly to the outer edges of the cake with a rubber spatula. (Note: It will look like the entire bottom of the cake is frosted.) Reserve any extra filling for garnish. Invert the cake onto a serving platter.
Using the extra filling, pipe rosettes around the top and bottom of the cake and place a malted milk ball in the center of each, if desired, or simply dust the cake lightly with cocoa powder (if dusting with cocoa powder, wait to do this until just before serving).
Chill at least 1 hour before serving.