Prep the pan: Preheat oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or grease well.
Mix dry ingredients: In a large bowl, whisk 2 cups fine almond flour, 2 tablespoons coconut flour, 2 teaspoons baking powder, 1/3–1/2 cup keto sweetener (adjust to taste), and 1/4 teaspoon salt until no lumps remain.
Whisk wet ingredients: In the same bowl, add 3 large eggs, 1/2 cup sour cream (or full-fat Greek yogurt), 1/3 cup unsweetened almond milk, 1/3 cup melted butter or avocado oil, and 2 teaspoons vanilla. Whisk until smooth and thick but scoopable.
Adjust consistency: The batter should be thicker than traditional muffin batter. If it looks dry, add 1–2 tablespoons more almond milk.
If too loose, add 1–2 teaspoons coconut flour and rest 2 minutes to thicken.
Fold in chips: Gently stir in 1/2–2/3 cup sugar-free chocolate chips. Reserve a few for the tops.
Fill the cups: Divide batter among the 12 liners, filling each about 3/4 full. Sprinkle the reserved chips on top for a bakery look.
Bake: Bake 18–22 minutes, until the tops are set and a toothpick comes out clean or with a few moist crumbs.
Avoid overbaking to keep them tender.
Cool: Let muffins cool in the pan 10 minutes, then transfer to a rack to cool completely. They firm up as they cool.
Taste and tweak: If you prefer sweeter muffins, dust the tops with a touch of powdered keto sweetener after cooling.