Preheat the oven to 180°C. Grease with a little butter and place baking paper in a circular mold of approximately 22 cm in diameter. Also, in a medium bowl, sift the flour with the cocoa powder, baking powder, baking soda and salt.
Add the instant coffee to the boiling water and stir with a spoon to dissolve. Set aside until needed.
Separately, in a large bowl add the eggs with the two types of sugar and beat with an electric mixer or a balloon whisk for 3-4 minutes until light and fluffy.
While continuing to beat, gradually add the oil with the vanilla extract and the buttermilk. Beat for 1 more minute until everything is perfectly integrated.
Add the flour mixture in two additions, beating for 20 seconds after each addition to obtain a smooth and homogeneous mixture. Finally, add the boiling coffee and beat just until it is completely integrated into the mixture.
Pour the mixture into the mold and spread it with the rubber spatula to make it uniform. Bake the cake at 180° C for approximately 35-40 minutes until a wooden toothpick inserted in the cake comes out clean.
Once the cake is ready, take it out of the oven, let it rest for 10 minutes inside the mold and then unmold it and place it on a wire rack to cool completely before decorating it.
For the frosting and assembly:
Melt the chocolate with the butter, which can be done in two ways. The first is in a double boiler, placing a medium saucepan with plenty of water over medium heat. Then, add both ingredients to a large heat-resistant bowl and place it on top of the pot with water (without touching the water) so that they melt with the steam.The second is in the microwave, adding both ingredients to a large microwave-safe bowl and then melting them for 30-second intervals, i.e., put the bowl in the microwave for 30 seconds and take it out after this time to mix a little with a fork. Repeat the process until both the chocolate and the butter are completely melted.
Once the chocolate mixture is ready, add the dulce de leche and mix with a rubber spatula until it is integrated and a smooth and creamy frosting is obtained. Cover the bitumen bowl with plastic wrap and refrigerate it for 30 minutes so that the bitumen takes more consistency.
When the sponge cake is cool, it can be cut in half with a bread knife to obtain two layers of sponge cake, or it can be left uncut. Once ready, place the sponge cake on a large serving plate.
If the cake was cut in two layers, fill it with the bitumen and then decorate it on top, spreading the bitumen evenly over the entire surface of the cake. If the cake was not cut, simply cover it evenly with the bitumen over its entire surface. Once ready, the cake can be eaten instantly or refrigerated for a few hours.