In a bowl, mix together the kosher salt, sugar, dark brown sugar, and crushed black peppercorns until well combined.
Spread a long piece of extra-wide aluminum foil on a flat surface and cover it with an equally long layer of plastic wrap.
Place half of the salt-sugar mixture on the prepared foil and wrap, and lay the salmon fillets on top, skin-side down.
Cover the salmon with the remaining salt-sugar mixture, then wrap tightly to fully enclose the fish and keep everything contained.
Refrigerate for at least 24 hours to let the salmon cure, then rinse off the curing mixture and pat the salmon dry.