To make the base; mix together crushed vanilla wafers, icing sugar, cocoa and butter by hand. Press ingredients into a 23cm springform pan.
In the top of a double boiler or bowl over simmering water, melt the chocolate chips, making sure that they are very smooth.
In a large bowl, beat cream cheese until fluffy with an electric mixer. Gradually beat in condensed milk until smooth. Mix in melted chocolate, eggs and vanilla. Beat with electric mixer on low speed until the ingredients are thoroughly blended.
Pour the filling into the prepared base.
Bake in preheated oven for 55 minutes. The cheesecake will seem underbaked in the centre, but will continue to cook after you remove it from the oven.