Make the Garlic Sauce: In a small saucepan, combine ½ cup heavy cream with 3 minced garlic cloves. Heat over medium-low heat for 5 minutes until fragrant. Remove from heat and whisk in ⅓ cup mayonnaise until smooth and creamy. Season with salt and pepper to taste.

Prepare the Patties: In a large bowl, combine 2 lbs ground beef with the remaining 3 minced garlic cloves, garlic powder, salt, and pepper. Mix gently until just combined. Form into 4 equal patties, making a slight indent in the center of each.

Cook the Patties: Heat a large skillet or grill pan over medium-high heat. Cook the patties for 4-5 minutes per side for medium doneness, or until they reach your desired level of doneness. Let rest for 2 minutes.

Toast the Buns: While the patties rest, slice the brioche buns in half and brush the cut sides with melted butter. Toast in a dry skillet or under a broiler until golden brown, about 1-2 minutes.

Assemble the Burgers: Place a handful of fresh arugula on each bottom bun. Top with a cooked patty, then generously drizzle with the creamy garlic sauce. Crown with the top bun and serve immediately while the sauce is still warm.