Preheat the oven to 180°C/160℃ fan/350°F/gas mark 4. Line a 23 cm/9 inch square cake tin with baking paper.
Place the chopped chocolate in a large bowl and pour over the boiling water. Stir until the chocolate is melted then whisk in the cocoa powder with a balloon whisk.
65 g 70% dark chocolate, 125 g (½ cup) boiling water, 40 g (½ cup) Dutch processed cocoa powder
Whisk the oil, yogurt, sugar and vanilla extract into the chocolate mixture.
96 g (7 Tablespoons) neutral oil (I use olive), 135 g (½ cup + 1 tablespoon) plain non-dairy yogurt (I use Greek style soya), 200 g (1 cup) light brown soft sugar, 2 teaspoons vanilla extract
Whisk in the espresso powder, baking powder, bicarbonate of soda and salt, followed by the plain flour. Stir just until no dry patches remain.
1 teaspoon instant espresso powder, ¾ teaspoon baking powder, ½ teaspoon bicarbonate of soda (baking soda), ¼ teaspoon table salt, 190 g (1 + ⅓ + ¼ cups) plain (all-purpose) flour
Pour into the prepared tin and bake for 20-25 minutes until a skewer inserted into the centre comes out clean or with a few moist crumbs but no wet batter. Leave to cool completely in the tin then store in an airtight container.