Use a serrated bread knife to cut the brioche into 8 slices, 3/4 to 1 inch thick. Lay them in a single layer on a wire rack and let sit uncovered for 1 to 2 hours, or overnight, to dry slightly.
Preheat oven to 200°F. Set a wire cooling rack over a baking sheet and place it on the middle oven rack to keep finished slices warm and crisp.
In a wide, shallow dish, whisk the eggs until fully combined. Add the milk, pumpkin puree, brown sugar, vanilla, pumpkin pie spice, cinnamon, and salt. Whisk for about 1 minute until completely smooth with no streaks or lumps.
Place a large skillet over medium to medium-low heat and warm for 2 to 3 minutes. Add 1 tablespoon butter and swirl until melted and foamy but not browning. Lower the heat if the butter browns instantly.
Place one slice of brioche in the custard and soak for about 15 seconds per side, pressing gently. Lift and let excess custard drip back into the dish. Soak only as many slices as will fit in your pan at one time.
Carefully place the soaked slices into the buttered skillet, leaving a little space between them. Cook for 3 to 4 minutes without moving until the bottoms are deep golden brown.
Flip each slice with a wide spatula and cook the second side for another 3 to 4 minutes. The center should spring back slightly and feel set rather than wet. Reduce heat if browning too fast.
Transfer cooked slices to the wire rack in the warm oven. Wipe out any dark bits from the skillet, add another tablespoon of butter, and repeat the soak and cook steps with remaining slices. Stir the custard before each batch.
Stack 2 slices on each plate while still hot. Dust with powdered sugar using a fine-mesh sieve, drizzle generously with warm maple syrup, and add a pat of butter if you like. Serve right away.