Topping (use first 6 ingredients): In a medium bowl, whisk flour, sugar, cinnamon and salt together. Add vanilla extract and melted butter into flour mixture, stirring until everything is uniformly moist.
Batter: In a large mixing bowl, beat butter and sugar until well combined. Add eggs, vanilla, and starter and mixing until smooth.
In a medium bowl, whisk together the flour, potato flour, baking soda, salt, and baking powder. Stir into the butter/starter mixture. Add the milk and mix until smooth.
Spread the batter in the prepared pan. Sprinkle the crumb mixture evenly over the top.
Bake cake for 45 minutes until a paring knife inserted in the center comes out clean. Remove from the oven and cool on a rack.