Preheat oven to 180 C / Gas 4. Line baking trays with baking parchment.
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 23cm long log. Place the logs onto the prepared baking tray, 10cm apart and flatten to 3.75cm thickness. If there is not enough room, put each one onto a separate tray.
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 1.25cm 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.
Can be purchased in health speciality shops or online.