Place miso in a small double boiler. Whisk in egg yolk, sugar, sake, mirin and matcha, if using, until smooth. Cook over simmering water, whisking, until the mixture begins to thicken, about 2 minutes. Gradually whisk in water and continue whisking until the sauce is thick enough to coat the back of a spoon, about 4 minutes. Scrape into a bowl and set aside.

Preheat broiler. Line a baking sheet with foil and coat with cooking spray.

Arrange salmon, skin-side down, on the prepared baking sheet. Dip your fingers into cold water and lightly moisten the top of the fish. Broil the salmon 4 to 6 inches from the heat source until just cooked through, 6 to 8 minutes, depending on the thickness of the fish.

Remove the salmon from the broiler. Put 1 tablespoon of the miso sauce over each portion, spreading evenly. Return the salmon to the broiler and continue cooking, shifting the baking sheet as necessary, until the salmon is cooked through and the topping is lightly browned, about 1 minute. Serve with lemon wedges.

Ingredient Notes: Miso: Fermented bean paste made from barley, rice or soybeans used in Japanese cooking to add flavor to dishes such as soups, sauces and salad dressings. A little goes a long way because of its concentrated, salty taste. Miso is available in different colors, depending on the type of grain or bean and how long it's been fermented. In general, the lighter the color, the more mild the flavor. It will keep, in the refrigerator, for more than a year.

Sake: A dry rice wine generally available where wines are sold. Junmai, a special designation for sake, denotes sake brewed from rice that has been milled less than other special-designation sakes. More pure than other sakes, junmai has no distilled alcohol added. It is characterized by a well-rounded, rich flavor and body and more acidity than most sakes.

Note: Matcha is an emerald-green powder made by grinding the older, carefully protected leaves of a Japanese tea plantation's oldest bushes. Some online sources are www.uptontea.com and www.edenfoods.com.

MAKE AHEAD TIP: Prepare through Step 1, cover and refrigerate for up to 2 days.