Preheat the oven to 180 C / Gas 4. Grease and line a 20cm (8 in) square baking tin.
Set a heatproof bowl over a pan of gently simmering water, ensuring the bowl doesn't touch the water. Add the chocolate and leave till melted and smooth, stirring occasionally. Stir in the coffee, soya milk and vanilla and remove from heat.
Combine the two sugars in a mixing bowl, then stir in the chocolate mixture and the egg.
Sift the flour, cocoa powder and bicarbonate of soda into the wet mixture and stir well. Finally, stir in the mayo. Pour the mixture into the baking tray.
Bake for 30 minutes. Feel free to keep checking with a skewer - it won't ruin like a cake! Take out once the skewer is clean.