For the sponge: Combine milk and yeast in the mixing bowl of a standing mixer. Whisk and let sit for 5 minutes. Add flour and sugar, mix to form a thick batter. Cover with plastic wrap and let rest at room temperature for 30-45 minutes, until bubbles form.
For the dough: Add the molasses, brown sugar, spice, flour, and salt to the sponge. Add the eggs. Beat with the paddle attachment on low speed for 2 minutes, until the eggs are absorbed. Increase the speed to medium and knead the dough for 5 minutes.
Switch to the dough hook attachment. The dough will be sticky. Turn themixer on medium low speed and add the butter, 2 tablespoons at a time. Knead for another 5 minutes, until the dough is shiny.
Scrape out the dough and clean and lightly oil the bowl. The dough may be a little difficult to work with. Place the dough in the bowl then turn it over so that the top is oiled. Cover with plastic wrap and let rise at room temperature for 2 hours, until doubled.
Punch down dough by folding it two or three times. Cover and refrigerate for 4 hours or overnight.
After the second rise, the dough is ready to be shaped. Turn the dough out onto a lightly floured surface. Using a rolling pin, roll it into a rectangle approximately 12x16 inches.
Filling: Use a pastry brush (or your hands if you like to get messy) to spread 3 tablespoons of butter around the rolled out dough, leaving an 1-inch margin at the top edge with no butter.
Combine sugar and orange zest in a small bowl. Sprinkle evenly over the surface, again leaving the 1-inch margin.
Starting from the long side closest to you, roll up the dough like a jellyroll. Transfer the log to a baking sheet, cover with plastic wrap and freeze for 10 minutes. This will make cutting the buns clean and even.
Remove from freezer, cut buns into about 1½-inch pieces. Place them back onto a greased baking pan. Spread out so they are not touching. Wrap with plastic wrap and let rise at room temperature for 1½ to 2 hours.
Preheat the oven to 400°F.
Bake buns for 10 minutes and reduce heat to 350°F. Bake for another 10-15 minutes depending on the size of your buns. Allow to cool on a wire rack for 10 minutes.
Make Glaze: Whisk sugar with orange zest and orange juice in a small bowl until smooth. Pour glaze over buns.