Oil an 8 x 8 baking pan and preheat your oven to 350 F.
Mix together the 1/4 cup reserved erythritol and the cinnamon in a small bowl and set aside for just before baking.
Mix together the remaining dry ingredients, blending well.
Add in the wet ingredients and stir to combine, distribute the topping over the batter, then bake for 1 hour. It should be bubbling in the center when it's done.
Cool completely, transferring to the fridge when the dish has cooled enough to do so. Cut and serve.
You can add extra topping if needed by mixing a little more cinnamon and erythritol together and sprinkling over the top before serving. But be careful, because this can become sickeningly sweet pretty quickly!
Please note that the nutrition data below is a ballpark figure. Exact data is not possible.