Preheat Oven to 180°c and line the base of the baking pan with parchment paper.
To prepare cake batter, melt 170 grams of the butter in a medium saucepan over medium heat, then add coffee and warm through.
Bring coffee mixture to a boil, remove from heat then whisk in cocoa powder until completely smooth. Let it cool for 1 minute.
Combine flour, sugar, baking soda and salt in a large mixing bowl. Whisk thoroughly to ensure everything is combined.
In a small bowl, whisk together eggs, sour cream and the vanilla extract until well blended.
Pour cocoa and coffee mixture into the flour mixture, then use a spatula or large spoon to gently stir until combined.
Add the egg and sour cream mixture and continue to stir until incorporated.
Pour the batter into two baking tins. Bake for 25 minutes at 180 degrees, or until a toothpick inserted in the center of the cake comes out clean.
Let the cake cool in the pans for 30 minutes, then invert the cake onto a rack to cool. Peel off the parchment.
(For the frosting, not in video) Heat the chocolate at high power in a microwave-safe bowl in 30-second intervals, stirring until most of the chocolate is melted then set aside to cool.
(For the frosting, not in video) In the bowl of an electric mixer fitted with a paddle, beat the butter at medium speed until pale and fluffy.
(For the frosting, not in video) Add the egg yolk and vanilla and beat for 1 minute, scraping down the side of the bowl.
(For the frosting, not in video) At low speed, slowly beat in the confectioners’ sugar, about 1 minute.
(For the frosting, not in video) In a small bowl, dissolve the instant coffee in 2 teaspoons of hot water.
(For the frosting, not in video) Slowly beat the coffee and the cooled chocolate into the butter mixture until just combined.
Set a cake layer on a plate with the flat side facing up.
Evenly spread one-third of the frosting over the cake to the edge.
Top with the second cake layer, rounded side up.
Spread the remaining frosting over the top and side of the cake.
Refrigerate for at least 1 hour before slicing.