Activate yeast (if using active dry): In a large bowl, combine 1/4 cup warm milk (105-115°F), 1 teaspoon sugar, and the yeast. Let stand for 5-10 minutes until foamy.

Combine wet ingredients: In a large bowl, combine sourdough discard, remaining sugar, salt, melted butter, and the activated yeast mixture (if using). Add the egg and whisk well.

Add dry ingredients: Gradually add the flour, mixing until a shaggy dough forms.

Knead the dough: Knead for 5-7 minutes until smooth and elastic.

First rise: Place the dough in a lightly oiled bowl, cover, and let rise in a warm place for 1-1.5 hours, or until doubled.

Prepare filling: Combine softened butter, brown sugar, and cinnamon.

Roll and fill: Roll dough into a 12x18 inch rectangle. Spread filling evenly. Roll up tightly and cut into 12 slices.

Second rise: Place rolls in a greased 9x13 inch pan, cover, and let rise for 30-45 minutes.

Bake: Preheat oven to 350°F (175°C). Bake for 25-30 minutes, or until golden brown.

Make glaze: Whisk together powdered sugar, milk, and vanilla extract. Drizzle over cooled rolls.