• Preheat the oven to  160°C Fan/180°C/350°F. 
• Lightly grease an 8-inch square cake pan. Line the bottom and sides with parchment paper.
• In a bowl, sift flour, sugars, cocoa powder, baking powder, baking soda and salt. Set aside.
• In a separate bowl or jug, whisk together eggs, oil, sour cream, milk and vanilla.
• Add wet ingredients to dry ingredients and whisk until just combined.
• Dissolve espresso powder in hot water and stir into the cake batter until incorporated,
• Pour batter into the prepared pan and place in the oven. Bake for 30-40 minutes or until a toothpick inserted into the cake comes out clean with a few crumbs sticking to it.
• Place the cake pan on a cooling rack and cool for 30 minutes before removing it from the pan and cooling completely. 

Chocolate Fudge Frosting

• Add chocolate callets to a heatproof bowl and melt at 30 second bursts in microwave. Set aside.
• Beat the butter on medium speed until pale and fluffy, about 5 minutes.
• Add in sifted icing sugar, cocoa powder, hot water, light corn syrup, vanilla and salt and beat until well mixed, about 1 minute. Increase the speed to medium and beat for 2 more minutes.
• Add the melted chocolate and beat on medium speed until smooth, about 1 minute.
• Fit a disposable piping bag with a 1M tip. Fill the piping bag with frosting and pipe rosettes on top of the cake.