Mix together the dry ingredients. Add the wet ingredients and mix with a hand mixer. The dough will be thick. Smooth into a small well-greased glass pan. I used a 4½ cup (1 L) pan that was about 6" x 7". This gave me a thick coffee cake. Feel free to use a bigger pan, but you'll probably have to decrease the baking time.
Combine the streusel ingredients with a fork. Crumble on top of the coffee cake and press it into the top a little.
Bake at 350 degrees F for 35-40 minutes or until a toothpick comes out cleanly and the middle isn't too mushy. Don't overbake it though. Let the coffee cake cool for a few minutes before cutting into 4 squares. I prefer to eat mine cold from the fridge.
Notes
Sweetener - I used ¼ tsp. of THM Pure Stevia Extract Powder to sweeten this coffee cake. Next time I would decrease the amount of stevia used and make up the difference with a granulated sweetener such as xylitol, Truvia, or THM Sweet Blend. Start with a little and increase to taste. Do not attempt to substitute another flour for the oat fiber or coconut flour unless you're a kitchen guru. Those two flours need special treatment with liquids and such. The carbs from the oats come out to a little over 5 g net carbs/serving. This is well within S guidelines for Trim Healthy Mamas.
Recipe by Briana Thomas at https://www.briana-thomas.com/cinnamon-streusel-coffee-cake/