Place the corned beef brisket fat side down in a crockpot, and then cover with water by 1-inch.
Sprinkle the brown sugar and peppercorns over the corned beef.
Cover with lid; cook for 8 to 10 hours on low heat.
Preheat oven to 350 degrees.
Carefully remove the brisket from the crockpot, scrape off the layer of fat, remove any peppercorns, and then place in an oven-safe baking dish.
Whisk together all ingredients until smooth.
Slather the cooked brisket with the sauce and bake for 10 minutes.