Preheat the oven to 180°C and line the muffin pan with 6 cupcake liners. Set aside.
In a glass, combine unsweetened almond milk and apple cider vinegar (or lemon juice). Set aside.
To the mixing bowl, add brown sugar and melted coconut oil. Mix with whisk. Then add vegan buttermilk and mix again. Sift all-purpose flour, ground cinnamon, cocoa powder, sea salt, baking soda, and baking powder. Mix until just combine. Don't overmix the batter.
Evenly divide the chocolate batter into 6 cupcake liners. Bake in preheated oven for 20 minutes until a toothpick inserted into the middle comes out clean. Otherwise, bake for 2-5 minutes longer.
After the cupcakes are baked, make sure to let them cool completely before frosting.
While the cupcakes are baking, fill ¾ of a small pot with water and bring to boil. Add finely chopped dark chocolate bar into a medium-sized glass bowl. Place the bowl on the pot, turn off the heat, and let the chocolate melt completely. Stir a few times with a spoon. Once melted, set aside.
Then, while stirring the chocolate, slowly add unsweetened almond milk (don’t add everything at once). Stir until combined into a smooth ganache. Add rum and a pinch of salt and stir again until combined.
Place the glass bowl with the ganache into the fridge for about 1 hour (to 1 hour and 30 minutes) until it thickens.
Add soft butter to a mixing bowl and beat with a hand mixer (or stand mixer) until smooth. Sift half of the icing sugar, mix until combine. While mixing, add 1 tablespoon at a time of almond milk and rum. Sift the rest of the icing sugar, and mix until soft, light, and super fluffy. It can take up to 2-3 minutes.
Us an apple corer or knife to remove the center of the cupcake. Be careful not to slice through the bottom. Fill each cupcake with about 1 teaspoon of the chilled dark chocolate ganache.
Then frost each cupcake with the buttercream frosting.
Decorate the cupcakes with mini marshmallows and dust with cocoa powder.
Serve them at room temperature or store in the fridge in an airtight container.