Cover potatoes with water; boil and simmer 15 to 18 minutes until soft. Drain well; steam-dry 3 minutes.
Heat oven to 350°F; grease a 2-quart dish. Melt filling butter with brown sugar in a saucepan. Remove from heat; carefully add bourbon.
Simmer on low 2 minutes. Mash potatoes with mixture, cream, salt, and cinnamon. Cool until warm; beat in eggs.
Spread in dish. Toss walnuts and panko with topping butter; scatter evenly.
Bake 30 to 35 minutes until crust golden and center reaches 160°F. Rest 10 minutes.