In a bowl, mix soft butter, salt, spices, herbs. Make a butter paste. Rinse and dry the duck with paper towels, put on foil. With a sharp knife score the skin on top 3 times on each breast. Spread butter on all sides salt and spices. If you want to put fresh rosemary and thyme on top do so now.
Cover with foil and leave for an hour. Preheat the oven to 400 F. Unwrap duck and place in a casserole dish (breast side up). For the first half hour, bake the duck without foil, then cover dish with foil and bake for another hour at 360 F.
While this is baking, make the glaze. Mix everything except cornstarch in a saucepan. Bring to a boil. Whisk starch in 2 tbsp of cold water until smooth and pour into a saucepan. Once the glaze has thickened, remove the saucepan from the heat.
The readiness of the duck can be checked like this - pierce the breast with a wooden skewer. If transparent fat flows, and temp reaches 165 in middle of thigh and breast, the duck is ready.
Remove the fresh herbs from the top of the breast. Brush duck with glaze and serve warm.