Prep your pan and oven: Line a 12-cup muffin tin with liners and lightly grease them. Preheat the oven to 350°F (175°C).
Mix dry ingredients: In a medium bowl, whisk almond flour, coconut flour, baking powder, salt, and granular sweetener until well combined.
Combine wet ingredients: In a large bowl, whisk melted butter, eggs, vanilla, and almond milk.
Add lemon zest if using.
Make the muffin batter: Add the dry mix to the wet ingredients and stir until just combined. If the batter seems too thick, add 1–2 tablespoons more almond milk. Gently fold in the blueberries.
Make the cheesecake mixture: In a separate bowl, beat cream cheese with powdered sweetener until smooth.
Mix in egg yolk, vanilla, and lemon juice until creamy.
Fill the cups: Spoon about 2 tablespoons of muffin batter into each liner, then add 1 tablespoon of cheesecake mixture on top. Finish with another tablespoon of muffin batter. Swirl gently with a toothpick.
Bake: Bake for 20–24 minutes, or until the tops are set and lightly golden and a toothpick inserted into the muffin portion comes out mostly clean.
The cheesecake centers will be slightly soft.
Cool completely: Let the muffins cool in the pan for 10 minutes, then move to a rack. Chill for 30–60 minutes for the creamiest texture.
Serve: Enjoy chilled or at room temperature. The flavor deepens after a few hours.