Make the marinade: Combine the miso paste, rice vinegar, brown sugar and soy sauce in a shallow bowl. Place the salmon into the bowl and make sure it's coated in the miso mixture. Cover the bowl with plastic wrap and marinate at least 1 hour.
Preheat oven to 400 degrees. Line a baking tray with parchment paper.
Cook the salmon filets in the oven for 10-12 minutes.
While the salmon is cooking, make the sauce: Melt the butter over medium heat in a large skillet. Saute the ginger and garlic in the butter until sizzling. Pour in the sake. Reduce heat and simmer 5-7 minutes.
Bring salted water to a boil in a medium saucepan. Set up a separate bowl of ice water. Blanch the snow peas in the boiling water for 2 minutes. Then, immediately plunge the snow peas into the bowl of ice water.
Divide the salmon and snow peas among 4 serving plates. Pour the sake butter sauce over top. Serve warm.