If you want to use cocoa powder instead of chocolate, you will need to change some ingredients as well.  The suitable ingredients for using cocoa powder will be: 1/2 cup butter, 5 tablespoons cocoa powder, a can of condensed milk (390g), and 400g digestive biscuit (crumbs). If you are going to use unsweetened cocoa powder, you may need to add 3 tablespoons of granulated sugar.