You need to preheat the oven to 180°C/350°F. Take a pan, grease it with parchment paper, and keep it aside.
Take a small bowl and add coconut flour, almond flour, salt, and baking powder to mix everything well.
Use a hand mixer to mix the sweetener and butter together. Then, add eggs into the mixture along with vanilla extract and mix until combined properly.
Now, take another small bowl to whisk the crumb topping ingredients well.
Add some of the cake batter to a pre-prepared baking pan. Then, spread the crumble mixture on top. After that, add the rest of the cake batter mixture to cover the crumble layer. Again, top it with the crumble mixture.
Put the coffee cake into the oven for baking for 35-40 minutes, or until a skewer comes out clean.
Take out the pan, and let it cool before serving.