Place the chicken breast in a shallow bowl and pour the buttermilk over it, letting it marinate for at least 15 minutes.
In a separate small bowl, whisk together the flour, cornstarch, garlic powder, onion powder, salt, and pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it thoroughly in the flour mixture until evenly coated.
Heat the avocado oil in a non-stick skillet over medium-high heat until shimmering.
Carefully place the chicken in the pan and cook for 5 to 6 minutes per side until the crust is crispy and the internal temperature reaches 165°F.
While the chicken cooks, whisk the Dijon mustard and honey together in a small ramekin.
Steam the broccoli florets until tender-crisp, about 4 to 5 minutes.
Drizzle the honey-mustard glaze over the hot chicken and serve immediately alongside the steamed broccoli.