Preheat oven to 350 degrees F. Grease a 12-cup bundt pan with nonstick cooking spray and set aside. Measure out 1/3 cup of the sweetened condensed milk for using in the cake. Reserve the rest for the icing. Combine flour, baking soda, salt, and cinnamon together in a medium bowl. Set aside. In the bowl of an electric mixer, combine the shortening, sugar, brown sugar, eggs, 1/3 cup sweetened condensed milk, and vanilla; beat at medium speed until creamy and fluffy. Add buttermilk and cocoa powder and beat until blended. Add flour mixture and beat on low to blend. Then increase the speed to medium and beat for 5 minutes. Stir in hot water with a spoon until just blended. Pour batter into prepared bundt pan and bake for 35 to 50 minutes, until a toothpick inserted in the center comes out clean. Cool for 5 minutes in the pan and then remove to a serving plate, fluted side up. Cool cake for 20 minutes.
Combine icing ingredients in the bowl of an electric mixer and beat at a medium speed until smooth. Stir the ingredients for the drizzle together until combined and set aside. Frost the warm cake with the icing and allow the hole in the middle to fill if you like. Sprinkle the marshmallows and pecans evenly over the icing. Combine the cocoa and vegetable oil. Decorate the cake with the chocolate drizzle. Serve warm or at room temperature. Lawd, this is one fantastic cake!