Grease a 15cm microwaveable silicone cake tin with a little oil and line the bottom with a circle of parchment paper.
Combine sugar, flour, cocoa, espresso powder, baking powder and salt in a bowl. In a bowl, whisk together oil, egg and hot water.
Add the wet ingredients to the dry ingredients, mixing well until a lump-free batter forms. Pour the mixture into the cake tin and tap a few times on the work surface to dissipate any air bubbles. Cover with cling film.
Microwave at full power (800W) for 5 minutes.
Take the cake out of the microwave and check if it is cooked: stick a skewer in the middle, and if it comes out clean, the cake is cooked. Let the cake rest for 5 minutes while you prepare the ganache.
To make the ganache, melt the chocolate at half power (600W) for 2 minutes, stirring often to prevent it from burning. Remove from microwave and add cream, stirring until smooth and glossy.
To decorate, melt the white chocolate in a small bowl on medium power (600W) for 1 minute, stirring halfway through, until smooth and runny.
Unmold the cake onto a serving platter and let cool for a few minutes.
Spread the ganache over the cake. Using a teaspoon, spread the white chocolate in thin parallel lines over the ganache. Slide a cocktail stick or skewer up and down, pulling through the chocolate for a feathery effect.