Prepare a round or circular tart pan of your choice. Melt chopped chocolate using 30 seconds spurts in microwave. After each spurt, gently stir the chocolate to check if it has melted. Be watchful in this step as chocolate burns very quickly. Once the chocolate is melted, set aside.

Place digestive biscuits in the jar of your mixer or processor. Grind them into coarse powder. Now add melted butter, coffee powder, cocoa powder and melted chocolate. Churn all of them together to make a thick sticky paste. Scoop this mixture into the prepared pan. Spread it evenly. Using the back of a spoon, press it down to make a uniform base. Allow it to set in refrigerator while you prepare the filling.

Repeat the process of melting chocolate from above using microwave for Mousse layer. Set is aside to come down to room temperature. In a chilled bowl, beat the whipping cream to soft peaks on slow speed. Now add icing sugar to it. Continue to beat till stiff peaks are formed, about 5-7 minutes total time. Now gently fold in melted chocolate to the whipped cream. Make sure the chocolate is not hot at all, else it will freeze into small particles and not make a smooth mousse. Also do not whisk or use rigorous mixing, the mousse will not form. Pour the mousse onto the set biscuit layer. Gently press and smoothen out the layer for even distribution. Regrigerate again for setting while you prepare ganache layer.

Roughly chop the chocolate and place in a heat proof bowl, keep aside. Heat cream in a thick pan on low heat, do not allow it to boil. Pour hot cream over chopped chocolate. Allow it to stand for few minutes. Using a spoon or a whisk, stir it gently till the chocolate melts. The heat of the cream will melt the chocolate. Allow the ganache to cool down and thicken slightly.

Pour the ganache over the set mousse layer. Top with cocoa powder, nuts, chocolate chips etc as per your choice. Allow the tart to set overnight or at least for 2 hours in refrigerator. When ready to serve, remove from refrigerator and slice it. Enjoy.