Preheat oven:

Heat oven to 350°F (180°C / gas mark 4).

Prepare donut pan:

Butter a donut pan or coat with nonstick spray, then dust with flour to prevent sticking.

Cream butter mixture:

In a large bowl, use an electric mixer to cream butter and confectioners' sugar until pale and fluffy, roughly 2 minutes. Mix in buttermilk, egg, lemon juice, and lemon zest until fully combined.

Fold in dry ingredients:

Add flour and baking powder; fold by hand just until a thick batter forms.

Pipe batter into pan:

Fill a pastry bag fitted with a large round tip with batter. Pipe into the prepared pan, filling each cavity 2/3 full.

Bake and cool donuts:

Bake 10-12 minutes, until a toothpick inserted near the center emerges clean. Let cool in the pan for 10 minutes, then turn out onto a wire rack.

Make lemon glaze:

As donuts cool, prepare glaze: In a shallow bowl, whisk sifted confectioners' sugar with 2 tbsp lemon juice until smooth. If needed, add more lemon juice to thin the glaze.

Glaze and garnish donuts:

Dip the tops of cooled donuts into the glaze, turn over, and return to the rack. Optionally, sprinkle with lemon zest. Allow glaze to set.