Preheat oven to 450F. Line a baking sheet with parchment paper. Boil water in a medium pot. Chop ends off of Brussels sprouts. Add Brussels sprouts to boiling water and blanch for 8-10 minutes. Once the Brussels sprouts are blanched, drain and place in an ice bath. Dry off Brussels sprouts as much as possible with a towel or paper towel.
While the Brussels sprouts are blanching, combine garlic, balsamic vinegar, olive oil, and salt in a small bowl.
Lay out Brussels sprouts on the baking sheet. Using the bottom of a jar or cup, gently smash the Brussels one-by-one. TIP: Place a towel or paper towel over the baking sheet to absorb any excess moisture; remove once you're done smashing.
Drizzle balsamic vinegar and oil mixture onto the Brussels sprouts. Toss well to evenly coat. Spread the Brussels sprouts evenly on the baking sheet. Bake in the oven for 15 minutes or until the Brussels sprouts are slightly browned. Remove from heat and serve immediately.