Broiler instead of kitchen torch

Use the broiler But avoid the closest rack which will probably singe the top of the pie. Use the second or third down rack setting.

Broil for 1-2 minutes until you see browning. Watch it the whole time as it can change quickly.

And/or look at other recipes for s’mores slab or sugar pie pan. Especially from highly regarded cooks, authors or bloggers. For example, Molly Yeh has a recipe.