Preheat oven to 400°F (200°C).

Melt butter in a small saucepan over low heat; remove from heat.

Whisk in 1/4 cup Dijon mustard into the melted butter.

Set aside the butter-mustard mixture.

Grind pecan halves in a food processor in small batches until a fine powder forms.

Place the ground pecans in a shallow dish.

In a separate bowl, combine the yogurt, remaining Dijon mustard, sliced black olives, and Hidden Valley Ranch salad dressing mix.

Season the chicken breasts with pepper to taste.

Dip each chicken breast into the yogurt mixture, ensuring it is fully coated.

Dredge the coated chicken breast in the ground pecans, pressing gently to adhere.

Place the pecan-crusted chicken breasts on a baking sheet.

Drizzle the reserved butter-mustard mixture over the chicken breasts.

Bake for 20-25 minutes, or until the chicken is cooked through and the crust is golden brown.

Garnish with a mint sprig before serving.