Preheat the oven to 350F and butter a 7x11 or 9x9" baking dish (ceramic works best for serving).
In a bowl, combine flour, sugar, salt, baking powder, and baking soda, ensuring there are no lumps.
Add eggs, oil, and buttermilk, whisking the ingredients together.
In a separate bowl, mix boiling water with cocoa powder and instant coffee until smooth and shiny.
Fold the cocoa mixture into the cake batter.
Pour the batter into the prepared dish.
For the fudge sauce, combine sugars and cocoa, removing any lumps. Sprinkle over the cake batter.
Gently pour boiling water evenly over the cake batter.
Bake for ~30-35 min until the top is firm but the bottom is delightfully fudgy.
Serve the cake hot with a scoop of vanilla ice cream – and indulge in chocolatey bliss!