Preheat oven to 190 degrees Celsius. Butter the bottom and sides of an 8-inch tin with butter and line the with parchment paper until the sides extend out, so that it's easier to lift the brownies out. Set aside.
In a heatproof bowl, place the butter and chocolate together. Place it over a pan of simmering water, ensuring the bottom of the bowl does not touch the surface of the water. Stir the mixture occasionally until all the butter and chocolate has melted. Remove from heat and stir in vanilla, coffee granules, Kahlua (if using) and salt. Set aside to cool for 15 minutes.
While the chocolate mixture is cooling, whisk the caster sugar, brown sugar and cocoa powder in a mixing bowl. Add in the egg whites and whisk until incorporated. Whisk in the cooled chocolate mixture. Sift the flour directly into the mixing bowl and whisk until incorporated.
Pour the batter into the prepared tin and place into the oven. Turn the heat down to 175 degrees Celsius and bake for 30 to 40 minutes, until a skewer inserted into the centre comes out with moist crumbs (do not over bake!)
Set the baking tin on a wire rack and let the brownies cool completely in the tin. Lift the brownies out, slice and serve.
For the best brownie experience - use the best chocolate that you can afford. I used Valrhona dark chocolate and Valrhona Dutch-processed cocoa powder.