Place the butter and milk powder in a medium saucepan over medium heat. Cook, stirring frequently, until bubbling and golden brown, about 3 to 5 minutes. Immediately transfer to a large mixing bowl to stop the cooking and set aside to cool for about 30 minutes or until it is starting to solidify around the edges.

In a medium bowl, whisk together the bread flour, cake flour, cream of tartar, baking soda, cinnamon and salt. Set aside.

Add the brown sugar and sugar to the cooled browned butter, beat until light and creamy.

Add the egg and vanilla, beat until well incorporated and smooth.

Stir in the sourdough discard until combined.

Add the flour mixture and stir just until all of the dry ingredients disappear. Cover the dough tightly and refrigerate for 2-3 hours or overnight.

Preheat the oven to 375°, line 2 baking sheets with parchment or silpats

In a small bowl, stir together the sugar and cinnamon.

Remove the dough from the refrigerator. Scoop ~2 tablespoons of dough (using a #40 cookie scoop), roll into a ball, roll the ball in the cinnamon sugar mixture and place on a prepared baking sheet. Repeat until all of the dough has been portioned out - leave about 2 inches between the dough balls on the baking sheet.

Bake for 10-12 minutes, the cookies will look soft and puffy, the edges will be lightly golden and the tops will be slightly cracked. Avoid over-baking, you want a slightly crisp edge and a soft center. Cool on the pans for a couple minutes before transferring to a wire rack to cool completely.