Whisk the pumpkin puree, milk, and active starter together until the starter is mostly dissolved.

Mix in the flour, brown sugar, salt, and pumpkin pie spice until a shaggy dough forms, then rest 20-30 minutes.

Add the softened butter and egg, then knead 8-10 minutes until smooth and elastic.

Bulk ferment 3-4 hours at room temperature, with 2-3 stretch and folds in the first 2 hours.

Shape into a loaf and place seam-side down in a greased 9x5-inch loaf pan.

Cover and refrigerate overnight, 8-12 hours.

Let proof at room temperature 1-2 hours until puffed and domed above the pan.

Bake at 350°F (175°C) for 45-55 minutes, tenting with foil at 25 minutes, until it reads 195-200°F internally.

Cool in the pan 15 minutes, then turn out and cool completely on a wire rack before slicing.