Place a heavy, rimmed baking sheet on the center rack and preheat the oven to 450°F so the pan gets hot.
While the oven heats, pat the Brussels sprouts very dry, trim the ends, and halve them (quarter if especially large).
In a large bowl, toss sprouts with olive oil, 1/2 teaspoon of the salt, and the black pepper until evenly coated.
Carefully remove the hot sheet, spread the sprouts cut-side down in a single layer, and return to the oven. Roast for 12 minutes without stirring.
Meanwhile, make the glaze: In a small saucepan over low heat, melt the butter. Stir in honey, balsamic vinegar, and red pepper flakes. Simmer gently for 30–45 seconds to thicken slightly, then remove from heat.
Shake or stir the sprouts, then continue roasting until the edges are deeply browned and centers are tender, 6–8 minutes more.
Transfer hot sprouts to a bowl, drizzle with the warm honey-balsamic butter, and toss to coat. Season with the remaining salt to taste and serve right away.