Spread a cup of mixed nuts at the bottom of a parchment paper-covered baking pan or tray.
In a small saucepan (on which you have installed a candy thermometer) heat the water, butter, salt, and sugars. Cook gently, until the thermometer reads 300°F (150°C). ATTENTION: the temperature will rise very quickly, so be attentive.
Remove from the heat immediately and add the vanilla (be careful, the mixture may splash a bit) and the baking soda.
Using a small spatula, promptly transfer the caramel to your nuts at the bottom of the pan.
Without waiting too long, spread the pieces of chocolate on the top of the caramel. Let the chocolate rest (& melt) for 2 minutes, then spread the chocolate over the nutty caramel in a thin layer.
Cover the chocolatey caramel with the other half of the grated nuts and press lightly with the palm of your hand to insert them a little into the caramel.
Let the toffee cool completely and break into pieces to serve.