Grind the cookies in a food processor and melt the butter on low heat. Mix the cookies with melted butter, cocoa powder, rum and milk.
Place the mixture into a 9-inch springform cake pan and press it on the bottom of the pan using your fingers to form the crust.
Cut the chocolate into smaller pieces and mix it in a small pot with 1/2 cup of heavy cream (unwhipped) and powdered sugar. Place the whole pot into a bigger pot, filled with water. Boil the water and stir the ingreedients in the upper pot until the chocolate and the sugar melts and dissolve and mix together completely. Set aside to cool.
Beat 1 cup of heavy whipping cream in a bowl with a hand mixer on medium-high to stiff peaks.
Using a hand mixer, mix the melted chocolate together with 1 1/4 cup (half of the amount) mascarpone cheese. With a spatula add the whipped cream to the bowl. The mixture should be smooth with no visible streaks.
Scrape the chocolate mixture into the pan and spread it evenly over towards the edges. Leave it in the refrigerator for at least one hour until the chocolate layer solidifies enough to continue adding the second layer to the cake.
Soak the gelatine leaves in cold water for 10 minutes until they become completely soft. Remove them from the bowl and squeeze out the excess water.
In a small pot, heat the blueberries together with the powdered sugar or sweetener until they get soft and looking like blueberry sauce. Blend the blueberries using a stick blender.
Add the gelatine leaves to blueberry sauce and stir until they completely resolve. Set aside to cool to room temperature.
Beat the remaining cup of the heavy cream.
Using a hand mixer, mix the reamining half of the mascarpone cheese with the blueberry sauce with gelatine. With a spatula gently mix in the whipped cream.
Scrap the blueberry mixture on top of the chocolate layer. Store it in the fridge for at least seven hours, preferably overnight, until both, the chocolate and the blueberry layer get strengthened and solid.
Decorate with fresh blueberries and chocolate balls or decoration of your choice.
Serve cold and enjoy :)