Prep the pan and oven: Preheat the oven to 350°F (175°C).
Lightly grease a 6- or 12-cavity donut pan with butter or coconut oil.
Mix dry ingredients: In a large bowl, whisk 1 1/2 cups almond flour, 2 tablespoons coconut flour, 1 1/2 teaspoons baking powder, 1/4 teaspoon salt, and 1/3 cup granulated keto sweetener until no lumps remain.
Combine wet ingredients: In the same bowl, whisk in 3 large eggs, 3 tablespoons melted butter, 1/4 cup Greek yogurt or sour cream, 2–3 tablespoons unsweetened almond milk, 1 tablespoon lemon zest, 2 tablespoons fresh lemon juice, and 1 teaspoon vanilla. Stir until smooth and thick, like muffin batter.
Adjust consistency: If the batter is too thick to pipe, add 1 more tablespoon almond milk. The batter should be scoopable, not runny.
Fill the pan: Spoon or pipe the batter into the donut cavities, filling each about 3/4 full.
Smooth the tops with a spatula.
Bake: Bake for 14–17 minutes, until the donuts are set, lightly golden at the edges, and a toothpick comes out clean. Do not overbake.
Cool: Let donuts cool in the pan for 5 minutes, then transfer to a rack to cool completely before glazing. They firm up as they cool.
Make the glaze: In a small bowl, whisk 3/4 cup powdered keto sweetener with 2–3 tablespoons lemon juice and 1–2 teaspoons almond milk until smooth and pourable.
Add extra lemon zest if you like.
Glaze the donuts: Dip the tops of cooled donuts into the glaze or spoon it over. Let them sit for 10–15 minutes to set.
Serve: Enjoy fresh, or chill for 15 minutes for a firmer glaze and enhanced lemon flavor.