Preheat the oven to 325°F/160°C.
Heavily grease the pan with butter and dust with cocoa powder.
In a large saucepan over medium heat, melt the butter, sugar, water, instant coffee, and sifted cocoa powder. Bring to a gentle boil.
Turn off the heat and add baking soda (the batter is going to rise for a second and that's OK).
Add the eggs and beat fast and strong (otherwise, you're going to end up with a chocolate omelet).
Sift together flour, baking powder, and salt and add to the mixture. Mix well until there are no lumps.
Pour the batter into the prepared cake pan and bake for 25-35 minutes until the cake is firm, and a skewer comes out almost completely dry (with moist crumbs). Cool completely before frosting.