Preheat the oven to 200°F.
Place the English muffin halves in a baking pan just large enough to hold them in a single layer.
Beat the pumpkin puree, milk, eggs, brown sugar, vanilla extract and pumpkin pie spice in a medium bowl. Pour the mixture over the English muffins, turning to coat. Let stand 20 minutes, turning once.
Heat a large nonstick skillet or griddle over medium heat. Melt half of the butter and add 4 muffin halves. Cook until the bottoms are lightly golden, about 3-4 minutes. Turn the muffin halves over and cook until lightly golden, about 3-4 minutes longer. Transfer the French toast to an ovenproof pan and hold in the preheated oven. Repeat with remaining muffin halves and butter.
Divide the French toast among 4 plates and serve drizzled with maple syrup.