For the donuts, pour milk into the bowl of your stand mixer fitted with dough hook. Sprinkle in yeast; let stand 10 minutes. Now add the donut’s zest, juice, sugar, butter, vanilla, egg and salt and mix to combine.
Add the flour and knead on medium speed until a loose dough forms that pulls away from the sides of the bowl. (The dough should be fairly soft, as that will ensure tender donuts.) Knead the dough 5 minutes.
Set the dough in a lightly oiled deep bowl. Cover with plastic wrap and let rise at warm room temperature until doubled in size, about 75 minutes.
Line 2 large baking sheets with parchment paper. Lightly flour a work surface and set the dough on it. Lightly flour a rolling pin and roll the dough until about 1/2” thick. Use a 3” biscuit cutter to cut the dough into rounds. Now use a 1 1/2” biscuit cutter to cut a hole out from centre of each round. Transfer the cut donuts and holes to the baking sheets, spacing them at least 3” apart. Gather up the scraps of dough and knead, roll and cut them into donuts too.
Cover the donuts with tea towels and let sit at warm room temperature 60 minutes. (Don’t worry if the dough does not double in size again, as it will puff in the oven.)
Preheat the oven to 375˚F. Bake donuts in the middle of the oven, one sheet at a time, 12 minutes, until puffed and very light golden. Cool donuts on a baking rack to room temperature.
To glaze the donuts, place the 1/4 cup juice in a shallow bowl and gradually whisk in the 2 1/4 cups of icing sugar. Dip and swirl the top of a donut into this glaze, and then lift up and let the excess drip away. Set donut, glazed side up, back on the baking sheet. Sprinkle the top of the donut with a little chopped pistachios and coconut. Repeat these steps with the remaining donuts.
To coat the donut holes, combine the 1/2 cup icing sugar and 1 tsp. orange zest in a medium bowl. Add the donut holes and toss to coat.
Note: You can warm the milk by zapping it on the microwave 15 to 20 seconds.