Preheat the oven to 350° F (180° C) and grease or spray a 10-inch springform pan or cake pan. Cut out a circle of parchment paper and use it to line the bottom of the pan.
In a medium bowl, stir together the all-purpose flour, spent grain flour, baking powder, salt, cinnamon, nutmeg, ginger, cloves, cardamom, and allspice. Set this aside.
In a large bowl, using an electric mixer, cream together the butter and brown sugar. Add the eggs one by one, mixing until each one is fully incorporated. Next, mix in the molasses.
Add 1/2 of the dry ingredients and mix until it they are only just incorporated, then do the same with the milk. Add the remainder of the dry ingredients and mix, scraping the sides of the bowl to make sure all ingredients are fully combined. Be careful not to overmix.
Pour the batter into the 10-inch pan and spread evenly. Peel and core the apples and cut them into slices about 1/4 of an inch in thickness. Layer the slices in rings on top of the cake, pressing the inner edges slightly into the batter.
Bake the cake at 350° F (180° C) for 50-60 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow it to cool for 1-2 hours before releasing the springform pan and glazing.