BA's Best Chicken and Dumplings

chicken legs (thigh and drumstick)

leek, pale-green and white parts only

carrots, 1 whole, 3 chopped into ½-inch rounds

celery stalks, 1 whole, 3 chopped into ½-inch pieces

medium onion, chopped, divided

garlic cloves, smashed

sprigs thyme

Parsley stems from ½ bunch

bay leaf

whole peppercorns

Kosher salt

chicken fat (schmaltz) or butter, melted, divided

Freshly ground black pepper

all-purpose flour, divided

baking powder

baking soda

plus 2 tablespoons buttermilk

heavy cream

Chopped chives (for serving)