Preheat the oven to 350°F (175°C) and prepare three 8-inch round cake pans by greasing them and lining with parchment paper.

In a large mixing bowl, whisk together the eggs, brown sugar, and sour cream until smooth and creamy.

In a separate bowl, combine the all-purpose flour, cocoa powder, baking powder, baking soda, and salt; sift and whisk together.

Gradually fold the dry ingredients into the wet mixture until just combined, careful not to overmix.

Dissolve the cocoa powder and instant coffee in 1 cup of boiling water and fold it into the batter until well integrated.

Pour the batter into the prepared pans and bake for 30-35 minutes or until a toothpick comes out clean.

Let the cakes cool in the pans for about 10 minutes before transferring them to wire racks to cool completely.

Prepare the salted caramel sauce by melting sugar in a saucepan until it turns amber, then add cream and salt and whisk until smooth.

In a mixing bowl, combine heavy cream, cocoa powder, and powdered sugar, mixing until smooth. Fold in cooled salted caramel and melted dark chocolate.

Once the cakes are completely cooled, stack them with layers of frosting and salted caramel in between.

Frost the top and sides of the cake using the remaining frosting and drizzle additional salted caramel on top.

Slice and serve at room temperature, optionally with whipped cream or vanilla ice cream.