Start by making your coffee mixture. Mix your espresso and rum together and place in the fridge for the mixture to get cold.
Now you need to make your mug cake, mix all ingredients together in a mug and microwave on high for 90 seconds. Take your cake out of the mug and let it cool down for 5 minutes. Slice your cake into 4 equal pieces and place them on a cooling rack to let them completely cool down.
On to the cream now. In a bowl, combine cream cheese, vanilla extract and erythritol. Using an electric hand mixer, combine all ingredients together until your sweetener has completely dissolved in the cream cheese and the texture of it is very smooth and creamy.
In a separate bowl, pour heavy whipping cream and whisk on high speed until medium to stiff peaks form, about 1 minute. Do not over beat.
Now add your whipped cream into the cheese mixture and whisk on low speed until the cream cheese is well incorporated into the cream. This should take 45 seconds to 1 minute.
Take your coffee mixture out of the fridge and dip your sliced almond cake into it. Make sure your cake is completely soaked. Now place one cake slice at the bottom of a cup/mug you're going to use to serve and scoop in 2 table spoons of the cream filling. Repeat this step one more time to finish building up your tiramisu cup. Drizzle some unsweetened cocoa powder on top of your dessert. Repeat the same steps for the second cup.
For best results, refrigerate for 2 hours before serving.