For the starter dough: Stir together all ingredients in a medium bowl. Cover and let stand at room temperature 4-5 hours.
For the final dough: Add the starter dough to a large bowl or the bowl of a stand mixer fitted with a kneading hook. Add the yeast, milk, sugar, salt, egg and butter to the bowl. Mix to combine. Add 2 cups of the flour and mix to combine. Add additional flour 1/4 cup at a time until you have a smooth, moist dough. Remove dough to a greased bowl, cover and let rise until doubled, about 45-60 minutes.
Meanwhile, combine sugar and lemon zest for filling in a small bowl and set aside.
Prepare a large baking sheets with parchment paper and set aside.
Remove dough to a floured surface and roll into a rectangle 12x16 inches. Brush dough with melted butter, then scatter sugar/zest mixture evenly overtop. Sprinkle poppy seed over-top. With the short edge of the dough closest to you, fold the top half of the dough down and over the bottom half, making an 12x8-inch rectangle. Using a sharp knife or a pizza cutter, cut the dough into twelve 1-inch wide strips.
Take one strip of dough and holding both ends, twist 5 or 6 times. Tie your twisted strip into a knot, then take the two loose ends and tuck them underneath, pushing one end up through the centre of the knot to secure. Place on parchment lined baking sheet. Repeat with remaining strips of dough. Cover baking sheet with a tea towel and let rise until puffy, 30-40 minutes.
Preheat oven to 375F (190C). When buns are puffy, bake in preheated oven for 20-25 minutes, or until golden. Remove to a cooling rack to cool completely.
For the glaze: Stir together icing sugar and lemon zest until smooth. Add additional juice/sugar as needed, to make a glaze that will easily fall off a spoon. Drizzle over cooled buns and allow to set.