Preheat your oven to 350°F. Grease a 7x11" or 9x9" baking dish with butter (a ceramic dish works well for serving).

In a large bowl, combine flour, granulated sugar, salt, baking powder, and baking soda. Mix well to remove any lumps.

Add the eggs, oil, and buttermilk to the dry ingredients. Whisk until well combined.

In a separate bowl, mix the boiling water with cocoa powder and instant coffee, whisking until smooth and shiny.

Pour the cocoa mixture into the cake batter and gently fold until combined.

Pour the batter into the prepared baking dish.

For the fudge sauce, combine granulated sugar, brown sugar, and cocoa powder, removing any lumps. Sprinkle this mixture evenly over the cake batter.

Carefully pour the boiling water over the entire cake batter, ensuring even coverage.

Bake for 30-35 minutes, depending on your pan size. The top should be firm and cooked, while the bottom remains fudgy and gooey. It's better to underbake slightly than to overbake.

Serve the cake warm with a scoop of vanilla ice cream, and enjoy!