Preheat the oven to 350°F.
Butter the inside surface of the bundt pan.
Combine the flour, sugar, baking powder and salt in the bowl of an electric mixer fitted with a flat paddle. Mix on low speed for 1 minute. Add the butter and mix for 1 minute.
In a separate bowl, whisk together the milk, eggs, egg yolks and vanilla. Pour one third of the milk mixture into the bowl and mix on medium-high for 1 minute. Add the remaining milk mixture; mix for 2 minutes. Scrape down the bowl and mix for 1 minute. Remove 2-1/2 cups of the batter and reserve.
In a microwave safe bowl, mix the instant espresso with 1 Tbsp. water. Add the chocolate sauce and unsweetened chocolate. Microwave for 30 to 40 seconds and stir until smooth. Add this mixture to the reserved 2-1/2 cups of batter, then add 1/4 tsp. baking soda.
Spoon half of the plain batter into the pan followed by half of the chocolate batter. Repeat the process with the remaining batters. Place the pan on the middle shelf of the oven and bake for 50 to 55 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Transfer the pan, bottom-side up, to a wire rack and cool for 15 minutes. To remove the cake from the pan, place the wire rack over the bottom of the cake, hold the rack firmly against the pan and invert. Place the wire rack on a baking sheet.
In a microwave safe bowl, combine the sugar, rum and 1/4 cup water. Microwave 35 seconds. Stir until sugar dissolves. Brush the hot syrup onto the cake with a pastry brush. Sprinkle the cake with sanding sugar. Cool the cake and serve.
Recipe is included with our Kitchen Essentials from Calphalon Bundt Cake Pan.