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1
In a medium pot, whisk the passionfruit juice, sugar, egg yolks, and salt to combine.
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2
Cook the mixture over low heat, stirring constantly, until the mixture is thick enough to coat the back of a spoon (don't let it boil!), 7-9 minutes.
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3
Remove the pan from the heat and stir in the butter. Stir until the butter is fully melted, then transfer the curd to a shallow dish and cover directly with plastic wrap.
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4
Chill in the refrigerator until very cold, at least 2 hours. Transfer the fully chilled (and thickened!) passionfruit curd to a disposable pastry bag and reserve, refrigerated.
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5
On a lightly floured surface, roll out your doughnut dough to about 1 inch thick.
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6
Use a 2 1/2-3 inch round cutter to cut circles out of the dough. These are filled doughnuts, so no need to cut another circle out of the center.
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7
Cover the doughnuts with lightly greased plastic wrap and let rise for 15-20 minutes. While the doughnuts are rising, heat about 4 inches of oil in large pot to 350 degrees Fahrenheit (or just tear off a scrap piece of dough and use it as a tester - if it bubbles and floats to the surface, you're good to go!).
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8
Fry the doughnuts until golden brown on both sides, 2-4 minutes per side. Drain on a rack set over absorbent paper towels.
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9
For best glaze results, let the doughnuts cool for 30 minutes. Once the doughnuts are cool, stick a paring knife into the side of each doughnut, then use that hole as a guide to fill with curd. Cut a small (1/4 inch) opening from the end of the disposable pastry bag, and stuff it into the doughnut as deep as you can go. Fill until the doughnut feels heavy in your hand. Repeat with all doughnuts.
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10
To make the glaze, whisk the powdered sugar and milk together to combine. You want a thick glaze, but not so thick that it won't easily cover the doughnut when dipped into it. Add more milk as needed to achieve the correct texture. Dip a spoon into the glaze - if the glaze clings fully to the spoon, but still drips off thickly, the glaze is perfect (if the glaze doesn't drip off the spoon, it's too thick - add more milk; if the glaze is very runny, whisk in more powdered sugar). Whisk in the orange zest and orange water, if using. When the doughnuts are cool, drop 1-2 drops of each color of food coloring onto the surface of the icing. Use a toothpick or skewer to swirl the color into the icing slightly.
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11
When the doughnuts are cool, drop 1-2 drops of each color of food coloring onto the surface of the icing. Use a toothpick or skewer to swirl the color into the icing slightly.
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12
Dunk each doughnut directly downward into the icing. Lift the doughnut out of the icing, and let the excess drip down for a few seconds.
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13
Quickly invert the doughnut onto a wire rack, allowing the glaze to drip off. The randomness of the color will create a lovely sunrise/sunset effect on the icing.
