Cocoa mixture: in a small bowl, whisk 1 tbs cocoa with 1/3 cup hot water and 2 tbs Kahlua rum
Place chocolate morsels into a medium sized, heatproof bowl and set aside.
Chocolate mixture: Place milk in a small saucepan over medium heat. Cook 2-3 minutes, stirring occasionally until you see tiny bubbles around the edge of the saucepan. Do not allow the milk to boil or it will burn.
Pour hot milk into the bowl of chocolate and allow it to site for 5 minutes, then stir until chocolate has completely melted and smooth.
Chocolate-pumpkin filing: In a separate bowl, whisk mascarpone until smooth. Add powdered sugar and whisk.
With a rubber spatula, fold in pumpkin puree until completely incorporated.
Fold in chocolate mixture and mix well.
Cut the dessert shells into 2 section.
Add the bottom shell on a plate. Liberally brush cocoa mixture and spoon pumpkin puree on top. Add the top part of dessert shell, brush with cocoa mixture, and top with pumpkin-chocolate mixture.
Refrigerate for 4 hours. To serve, dust tiramisu with cocoa powder and more chocolate.