Place all the chocolate sauce ingredients (sugar, sifted cocoa powder, water, butter, vanilla, rum) into a small saucepan. Turn on the heat at medium and let the ingredients come to a boil. Then let the sauce simmer for around 5 minutes and stir often. Remove from heat and let the sauce cool down.
In a heat proof medium sized bowl, heat up the heavy cream until it just starts to boil (1-2 mins). Place the dark chocolate chips in the heated cream and let sit for about 5 minutes. Stir the cream and chocolate until smooth and lump free. Set aside to cool.
Line a 25cm (10inch) springform cake tin with baking paper on the bottom and lightly spray the sides with oil. Roughly chop up the almonds and biscuits and place into a large bowl. Set aside around 2 tablespoons worth of chopped almonds to use later as topping.
When the chocolate sauce is lukewarm, add it to the large bowl with the chopped biscuits and almond. Mix well until all the biscuits and almonds have been coated evenly with the chocolate sauce.
Place the biscuit mixture into the prepared springform cake tin. Press down on the biscuit mixture so that the biscuits are tightly packed and so that you get a flat, levelled surface on top.
When the dark chocolate ganache is lukewarm, pour it over the top of the biscuit layer. Spread the ganache evenly over the top. Place in the fridge, covered, for around 3-4 hours, even overnight.
Top with the remaining chopped roasted almonds. Best served on the day it is made as the roasted almonds stay crunchy. Keep refrigerated in an airtight container for up to 5-7 days.
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.