Combine the sourdough starter, warm milk, and sugar in a bowl. Let it sit for a few minutes until it's bubbly.
Mix in the melted butter and egg to the sourdough mixture.
Gradually add the flour, salt, and baking soda. Knead until the dough is smooth and elastic.
Place the dough in a greased bowl, cover it, and let it rise in a warm place for about 1 hour or until it has doubled in size
Roll the dough into a rectangle on a floured surface.
Spread the softened butter evenly over the dough and sprinkle with a mixture of brown sugar and ground cinnamon.
Carefully roll up the dough, starting from one of the longer sides, to create a log.