Preheat the oven to 350 degrees F and line a 9” x 9” baking pan with parchment paper.

In a bowl, stir together the almond flour, coconut flour, baking powder, and sea salt until well combined.

Add the melted butter to a stand mixer fitted with a paddle attachment, along with the sugar-free granulated sweetener. Beat until well combined. Note: you can make the coffee cake in a mixing bowl by hand if you don’t have a stand mixer.

Add the eggs and vanilla extract to the mixer and beat on medium-high speed until well-combined and a creamy, custard-like mixture forms.

Add the flour mixture to the mixer and beat on medium-high until completely combined. Note: The cake batter will be very thick. This is normal!

Gently fold in the blueberries until they are well-distributed throughout the dough.

Stir together the ingredients for the streusel topping in a mixing bowl until well-combined.

Transfer half of the cake batter to the parchment-lined baking pan and press it into an even layer (I do this with my hands).

Sprinkle half of the streusel topping evenly over the cake batter. Repeat with the other half of the cake batter and the remaining topping.

Cover the cake pan with foil and bake on the center rack of the preheated oven for 30 minutes (the foil keeps the topping from burning). Remove the coffee cake from the oven, discard the foil, then place the uncovered coffee cake back in the oven to continue baking for another 20 to 25 minutes, or until the coffee cake feels firm when gently poked in the center.

Allow the coffee cake to cool at least 20 minutes before slicing and serving.