Preheat the oven to 190°C (375°F).
Mix the flour, cocoa powder, baking soda, salt and sugar in a bowl.
Add the oil, water, espresso powder (if using) and vanilla to the bowl.
Mix together with a whisk until smooth.
Add the vinegar and stir quickly. You'll see the batter change colour as the vinegar and baking soda react. Stir just until the vinegar is evenly distributed throughout the batter.
Pour into an 8 inch square or 9 inch round baking tin. I always grease and flour the tin to be on the safe side but the original recipe calls for an ungreased tin.
Bake for 25-30 minutes and then set on a cooling rack.
For the glaze, melt the chocolate in a pan over very low heat.
Stir the hot water and vanilla into the chocolate and stir until smooth. It may appear lumpy at first but keep stirring and it will smoothen out.
Spoon the glaze over the cooled cake. Refrigerate the glazed cake for at least 30 minutes before serving.