1. In a large, heavy skillet over medium-high heat, melt 2 tablespoons butter.
2. Add ¼ of the mushrooms and sprinkle with salt and pepper; do not crowd the mushrooms. Sauté until tender and starting to brown, 3–4 minutes. Transfer mushrooms to a bowl.
3. Repeat, using 2 tablespoons butter and ¼ of the mushrooms for each additional batch.
4. In a medium saucepan, bring chicken broth to a simmer; keep warm.
5. In a large, heavy skillet over medium-low heat, melt remaining 2 tablespoons butter with olive oil. Add leek and a pinch of salt; sauté until tender, 4–5 minutes.
6. Add rice and move skillet towards the center of the French top to increase heat to medium. Toast rice, stirring constantly, until edges of rice become translucent, 3–4 minutes.
7. Add white wine and vermouth; stir until liquid is absorbed, about 1 minute.
8. Add ¾ cup warm chicken broth; stir until broth is mostly absorbed, about 1 minute. Repeat, adding ¾ cup of broth at a time, stirring until broth is mostly absorbed before adding more, until rice is halfway cooked, about 10 minutes.
9. Stir in sautéed mushrooms.
10. Add broth ¾ cup at a time, stirring until broth is mostly absorbed before adding more, until rice is tender but still firm to bite and risotto is creamy, about 10 minutes.
11. Stir in ¼ cup grated Parmesan cheese. Transfer risotto to serving bowl. Pass additional Parmesan cheese alongside, if desired.