Preheat the oven to 350°F. Grease a 9" square pan., To make the topping: In a medium bowl, whisk together the flour, sugar, cinnamon, and salt. Add the vanilla and almond extracts to the melted butter and pour the butter into the flour mixture, stirring until you have a uniformly moist mixture. Set aside while you make the batter., To make the batter: In a large mixing bowl, beat the butter and sugar until well combined. Add the eggs, vanilla, and starter, mixing until smooth and scraping the bowl's sides and bottom as you go., In a medium bowl, whisk together the all-purpose flour, potato flour, baking soda, salt, and baking powder. Add to the butter/starter mixture, stirring until evenly combined. Add the milk and mix until smooth., Spread the batter in the prepared pan. Sprinkle the crumb mixture evenly over the top., Bake the cake for 45 to 50 minutes, until a paring knife inserted in the center comes out clean, and the edge of the cake just barely pulls away from the pan. Remove it from the oven and cool on a rack; dust the top with confectioners' sugar., Store the cake, the pan covered with plastic, at room temperature for several days; freeze for longer storage.