Garnishes: toasted sesame seeds, thinly sliced green onions
Thoroughly towel-dry salmon fillets to remove excess moisture. Set aside.
Using a hand-whisk, gently mix together the miso, sugar, Sake, Mirin, and pepper until smooth and fully incorporated. Dip each salmon fillet into miso mixture to evenly coat both sides of each piece of fish. Place salmon fillets in a single layer in 9x13 or other large pan; gently spread remaining miso mixture evenly on top of each fillet. Cover with plastic wrap and chill overnight, up to 24 hours.
Preheat broiler on high, with rack 8 inches from heat source. Set a wire rack on a large baking sheet. Line wire rack with foil. Gently remove excess miso mixture from fillets (do not rinse) and place fish on foil-lined wire rack. Do not overcrowd the fish (should be at least 1 inch space in between each piece.)
Broil salmon until browned on top/sides and centers feel somewhat firm upon lightly pressing down with a spatula (roughly 8 minutes, depending on thickness and temp of fish to begin with.) If tops/side brown too quickly, loosely cover with foil and cook until fish is just done (take care not to over-cook.) Garnish as desired. Serve while warm, with white or brown rice.
Try to marinate salmon overnight, up to 24 hours, before broiling. We used white miso paste for a milder flavor and appearance, but any color miso paste can be substituted. Use salmon fillets of even thickness, for even cooking results. Leftover salmon is delicious reheated, or served cold with a salad.