In a mixing bowl, combine the rock salt, granulated sugar and dark brown sugar until evenly mixed.
Lay a large piece of clingfilm on your work surface. Spoon about half of the sugar-salt mixture onto the clingfilm and shape it into a mound roughly the size of the salmon fillet.
Place the salmon fillet on top of the sugar mound, then cover the top of the salmon with the remaining sugar mixture, pressing to coat evenly.
Wrap the salmon tightly in the clingfilm and place it seam-side up in a large non-metal dish.
Place a small chopping board or plate on top of the wrapped salmon and add weights (such as cans) to press it; refrigerate for 12 hours.
After 12 hours, turn the wrapped salmon over, replace the board and weights, and refrigerate for another 12 hours.
Unwrap the salmon and scrape off any remaining curing mixture. Rinse the salmon well under cold water and pat the flesh dry with paper towels.
Cut the salmon into thick fingers and thread each piece onto a cocktail stick or small skewer.
Heat a flat grill pan over high heat until smoking. Sear one side of each salmon skewer briefly until lightly browned; remove from heat.
Serve the skewers immediately with soy dipping sauce if desired.