Preheat your oven to 350°F (175°C). Grease a 12-cup mini muffin pan or a donut hole pan well.
In a large bowl, whisk together the gluten-free flour blend, granulated sugar, baking powder, salt, and poppy seeds.
In a separate medium bowl, whisk together the milk, eggs, melted butter, vanilla extract, lemon zest, and lemon juice until combined.
Pour the wet ingredients into the dry ingredients. Mix gently until just combined. Do not overmix the batter.
Spoon the batter into the prepared mini muffin cups or donut hole cavities, filling each about two-thirds full.
Bake for 10 to 12 minutes, or until a toothpick inserted into the center comes out clean.
Let the donut holes cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
While cooling, prepare the glaze: Whisk the powdered sugar and 2 tablespoons of lemon juice together until smooth. Add more lemon juice, a few drops at a time, if the glaze is too thick.
Dip the tops of the cooled donut holes into the glaze, or drizzle the glaze over them. Let the glaze set before serving.
Notes
Use a gluten-free flour blend that contains xanthan gum for the best structure. If your blend does not contain it, add 1/2 teaspoon of xanthan gum to the dry ingredients.
For a stronger lemon flavor, use two lemons for zest and juice.
These donut holes freeze well; store them in an airtight container and thaw at room temperature.