For the chocolate biscuit cake: Line the bottom and sides of a 9 inch spring form pan with plastic wrap or parchment paper. Set aside. In a large bowl, break up the digestive biscuits into smaller pieces. Set aside.
In a large saucepot, combine the sugar and cocoa powder. Whisk to incorporate and break up any large pieces of cocoa. Gradually add your milk to the sugar and cocoa mixture, whisking well after each addition. Add the butter to the pot as well.
Place the pot over medium-high heat. Cook, stirring constantly, until the butter is melted and the mixture starts boiling. Reduce heat to medium, and simmer the mixture for about 5 minutes, stirring often, until it thickens enough to coat the back of a spoon. Remove the pot from the heat and whisk in vanilla extract. Let the chocolate sauce cool for 10 minutes.
Pour the chocolate sauce over the biscuit pieces. Stir until all of the biscuits are coated in the mixture. Transfer the chocolate/biscuit mix to your prepared spring form pan. Press the mixture down to compress the cookies together and to smoothen the top. Cover, and chill for at least 4 hours to set the cake.
For the chocolate ganache: Place the chopped chocolate in a medium sized bowl. Set aside.
Pour the cream into a small saucepot, and place the pot over medium heat. Warm the cream until it just barely starts to form bubbles along the sides. Don't boil! Pour the hot cream over the chocolate, and let sit for 5 minutes to steep. Gently mix until smooth. ** (see notes)
Pull your cake out of the pan, and transfer to your serving plate or stand. Pour the ganache over the top, and spread to cover the top of the cake. Slice and serve!