Melt the chocolate
Preheat the oven to 190°C / 375 °F. Melt the chocolate and butter in a bowl set over a saucepan of simmering water. Add the freshly brewed strong espresso and salt to the chocolate and butter mix, stir well and remove from the heat to cool.
Beat the eggs and sugar
Beat the eggs and sugar until thick and pale, about 5 minutes in a bowl set over a saucepan of simmering water.
Pour the chocolate mixture into the egg mixture and beat for another 2 - 3 minutes. Add the hazelnut flour and unsweetened cacao and stir well, using a spatula.
Prepare the glaze. In a saucepan bring the sugar, cacao and freshly brewed strong espresso to a boil. Simmer for 2 minutes, stirring constantly. Set aside until needed.
bake and serve
Grease the cake pan with butter. Lower the oven temperature to 175°C / 350 °F. Pour the batter into the cake pan and bake until set, for about 40 minutes. Serve with cacao glaze and/or vanilla ice cream.