Preheat oven to 180°C (350°F). Line a 9-inch square cake pan with parchment paper. Set aside.
Prepare the cinnamon streusel
In a medium-size mixing bowl, combine all the streusel ingredients. The order doesn't matter until it forms a cinnamon crumble. Set aside at room temperature while you make the cake batter.
Prepare the coffee cake batter
In a large mixing bowl, stir together almond flour, coconut flour, baking powder, erythritol, and salt.
In another medium-size bowl, beat eggs with melted butter (lukewarm not hot, or the butter will 'cook' the eggs), unsweetened almond milk, and vanilla.
Pour the liquid ingredients onto the dry ingredients and combine with a spoon until it forms a thick batter with no lumps. You can also use an electric beater, on low speed if desired.
Fill in the cake pan
First, spread evenly half of the coffee cake batter into the prepared cake pan.
Sprinkle half of the cinnamon streusel on top of the cake batter, then spread the rest of the coffee cake batter on top of the streusel. As you spread the rest of the cake batter, it will mix some of the streusels you added below, and that is ok.
To finish, sprinkle the rest of the cinnamon streusel all over the top of the pan. Slightly press with a spatula to stick the streusel to the cake batter.
Bake 30-40 minutes or until the cake is golden and a skewer inserted in the center of the cake comes out clean.
Cool it down for 10 minutes in the cake pan, pull the parchment paper pieces to lift out the cake from the pan easily, and transfer onto a cooling rack for 1-2 hours or until it reaches room temperature.
When the cake is at room temperature, drizzle the sugar-free glazing on top of the cake. To make the glazing, combine the sugar-free powdered erythritol and unsweetened almond milk into a small bowl until it reaches your desired thickness. Add a bit more powdered erythritol for thicker glazing.
Cut into 16 squares and serve for breakfast or snack, with a dollop of unsweetened whipped cream (or whipped coconut cream, if dairy-free) and a cup of coffee.
Store in the fridge in a cake box for up to 3 days or freeze for 3 months. Defrost 4 hours before at room temperature.