Preheat oven to 400F. Liberally salt beef with kosher salt on both sides. Rub matcha powder on one side of beef and drizzle with 2 tsp vegetable oil or melted butter. Add green peppercorn to one side of beef. Heat 1 Tbsp vegetable oil in cast iron pan or medium saute pan over high heat until slick and shiny. Add beef, peppercorn and matcha rubbed side down, and sear 30 seconds. Lower heat to low and let sear another minute. Transfer beef to parchment-lined oven-safe sheet tray and bake 8-15 minutes, depending on desired doneness.
Drain pan of excess oil. Add shallots and peppercorn and saute on low heat 1-2 minutes until shallots are translucent. Deglaze pan with sake or water and let liquid reduce to approximately 1 Tbsp. Strain sake liquid. Place matcha powder in medium bowl and whisk in sake liquid until evenly combined and frothy. Transfer matcha sake liquid back to saute pan and reduce heat to low. Add butter, a few small cubes at a time, while whisking continuously to create an emulsion. Season with salt to taste. Keep warm for use, or sauce will begin to break.
Remove beef from oven and let rest 5 minutes. Slice and serve with matcha sauce.