Preheat oven to 180 degrees C. Line baking trays with baking paper.
In a large bowl beat the sugar, olive oil and margarine until smooth. Stir in the egg substitute and vanilla. Combine the flour, cocoa and baking powder, gradually stir into the wet ingredients. Mix in the chopped chocolate. Chill dough for 10 minutes.
Divide the dough into two equal pieces. Roll each piece into a 25cm long log. Place the logs onto the prepared baking tray 4 inches apart and flatten to 4cm thickness. If there is not enough room, put each one onto a separate sheet.
Bake for 20 to 25 minutes in the preheated oven until somewhat firm. Cool on the baking trays until cool enough to handle.
Remove baked loaves to a cutting board and cut each one crosswise into 1cm slices with a sharp knife. Place the slices cut side down back onto the baking trays and toast on each side for an additional 8 to 10 minutes. Cool on wire racks and store in an airtight container.