Peel and cut the potatoes into 1-inch chunks, add to a pot of cold water, bring to a boil over high heat, then reduce to medium and cook for 15 to 18 minutes until fork-tender. Drain and let steam for 1 to 2 minutes.

Return potatoes to the warm pot over very low heat, mash with 3 tbsp butter, stir in the warmed milk gradually, then add ¾ tsp salt and ¼ tsp white pepper. Keep covered on low heat.

In a medium saucepan over medium heat, melt the remaining 1 tbsp butter, whisk in the 2 tbsp flour, and cook for 1 to 2 minutes until the roux turns light tan and smells nutty.

Slowly pour in the 1¾ cups beef broth and ¼ cup chicken broth while whisking, add ½ tsp onion powder, ½ tsp garlic powder, ¾ tsp black pepper, and ½ tsp Worcestershire sauce, bring to a gentle boil over medium-high heat, then reduce to medium-low and simmer for 4 to 5 minutes. Stir in the cornstarch slurry (1 tbsp cornstarch mixed with 2 tbsp cold water) and cook for 1 to 2 more minutes until thick. Season with salt to taste.

Scoop mashed potatoes into bowls, ladle gravy over the top, and finish with a pinch of black pepper.