Preheat oven to 170 degrees C and prepare a small brownie tray with baking paper or grease.
Add cocoa and salt to the melted butter and stir till smooth (if using coffee powder add now).
Whisk eggs and sugar till combined. Add espresso, vanilla and flour and stir till combined to a smooth batter. Add cocoa mixture and combing till the mixture is the same shade (no residue of cocoa mixture in batter).
Add any desired mix ins then pour batter into prepared tin. Bake for 30 mins. Test by inserting a skewer, it should still be slightly wet but not completely, return to oven if it still looks like batter.
Enjoy warm from the oven with maybe a glass of milk or a scoop of ice-cream! It's just as delicious the next day without anything at all!