Pre-heat oven to 350 degrees. Line a 9x9" baking sheet with parchment paper. Set aside.
Mix all dry cake ingredients, then add wet ingredients and mix until combined. Pour into lined baking sheet and spread evenly. Bake for 12-15 minutes or until a toothpick inserted into the center of the cake comes out clean.
Remove cake from baking sheet to cool. If you'd prefer to keep the cake from getting too soft when you soak it in the rum, you can let the cake sit out overnight to dry out a bit. I made mine right away.
Using a hand mixer or immersion blender, mix all "pastry cream" ingredients until smooth. Set aside.
To cut the right size circles, turn your serving cups upside down on the cake and press hard enough to make an indent. Follow along the indients with a knife to cut out 6 cake rounds. Depending on the size of your serving cups, you may only get 4 perfect rounds and need to use the cake scraps to assemble your 3rd cup.
Spoon a small amount (~1 tbsp) of pastry cream into the bottom of your serving cups - just enough to cover the bottom.
Add rum and espresso to a small bowl. Quickly dip a cake round into the liquid and press into the serving cup on top of the pastry cream. Top with another layer of pastry cream. Repeat process until there are two layers of cake and 3 total layers of pastry cream per cup.
If desired, allow to set up in the refrigerator overnight. Sift dutch cocoa over top of the cups before serving.
For even creamier pastry cream, use full fat coconut milk.
I used these 10 oz mini trifle dishes, which I think are perfect for sharing. This recipe will stretch to 4 servings if you use 8 oz serving cups instead.