makes one 8x8-inch pan, 12 small bars
Preheat the oven to 350°F. Butter and flour an 8-inch square baking pan.
Melt the butter in a small saucepan then set aside to cool as you prepare the apples and toast the walnuts, if you need to do that.
Add the sugars to the butter and stir. Stir in the egg.
In a bowl, whisk together the flour, cinnamon, baking soda, salt, allspice, and nutmeg. Pour the butter into the dry ingredients and stir until just combined.
Add the diced apple and toasted walnuts. Spread into prepared baking pan and bake for 35–40 minutes until top is crisp and edges are beginning to dry out.