Lightly oil (or butter) a non-stick 20 cm springform pan; if yours is not non-stick, butter the bottom, line bottom with parchment paper, then butter the sides and the parchment as well.
Melt the chocolate and butter in a heat-proof glass bowl over a pot of simmering water, stirring with a wooden spoon until both are completely melted and the mixture is smooth; take the bowl off the heat (and off the pan of water).
Mix the instant coffee granules and 3 TBS cocoa powder into the chocolate mixture and set aside.
In a large bowl, beat the eggs and the granulated sugar together for five minutes at high speed, until they are light in color and thickened.
Grind 30 g of the walnuts in a spice or coffee mill until they become a fine powder.
Coarsely chop the remaining walnuts.
Stir the ground walnuts and walnut pieces into the egg mixture.
Carefully fold the chocolate mixture into the egg mixture until they are completely combined (I like to use a glass bowl so I can see that I've done so).
Pour the mixture into the prepared pan.
Bake 40 minutes or until the top of the cake is dry.
Turn the oven off, leaving the cake undisturbed IN THE OVEN to cool COMPLETELY.
When the cake is cooled, remove from the oven.
Remove the cake from the pan, remove parchment paper if used, and sprinkle with the reserved cocoa powder and powdered sugar.