Pulse butter, kimchi and kimchi juice, and salt in a food processor, scraping down sides as needed, until kimchi is finely chopped and fully incorporated into butter. Cover and store at room temperature.
Kimchi butter can be made 3 days ahead. Chill.
Chicken
Grind coriander seeds, red pepper flakes, lemon zest, black peppercorns, cumin seeds, and 4 tsp. salt in a spice mill or with a mortar and pestle until very finely ground.
Place chicken, skin side up, on a rimmed baking sheet. Using the heel of your hands, press firmly on breastbone to flatten. Season chicken on both sides with ¼ cup spice mixture, patting to adhere. Chill, uncovered, 3–8 hours.
Boil potatoes in salted water until just tender, 15–20 minutes. Remove from heat and let sit uncovered until ready to use (up to 5 hours).
Preheat oven to 550° (or maximum setting). Place chicken on a foil-lined rimmed baking sheet, leaving wings untucked. Place half of the kimchi butter in small pieces over the chicken and roast until browned but not fully cooked, 20–25 minutes. Arrange bacon, potatoes, and corn around chicken; sprinkle with reserved spice mixture. Roast until an instant-read thermometer inserted into the thickest part of the thigh registers 165°, 10–15 minutes. Transfer to a cutting board and let chicken rest 10 minutes before carving.
Transfer bacon and vegetables to a large bowl and add remaining kimchi butter; toss to melt butter and coat vegetables. Serve alongside chicken.
