Stir flour, pumpkin, egg, sugar, salt, and pumpkin pie spice together in a large bowl until well mixed; the mixture will be thick and chunky. Add baking powder and stir until evenly distributed. Slowly pour in 1/4 cup milk while stirring; mix until all liquid is absorbed. Add more milk and stir until batter is smooth and firm, yet flows off the spoon; a wooden spoon inserted into the mixture will stand, but batter will flow when poured. Mix in pecans.
Heat a nonstick skillet over medium heat. Scoop 1/4 cup batter for each pancake onto the skillet. Cook until edges are dry, 1 to 2 minutes; flip and cook on the other side until edges are firm, another 1 to 2 minutes.
Place a pat of butter on each pancake as they are removed from the skillet and transferred to a plate. Sift confectioners' sugar on top.
