In a small bowl, mix together the milk, sugar, and yeast. Allow to sit for 5 minutes, or until the yeast begins to bubble and foam.
In a standing mixer bowl fitted with a dough attachment, combine flour and salt. Add in egg, vanilla extract, melted butter, and milk mixture, stirring to combine.
Knead dough on low speed for 10 minutes, until the dough is soft and very stretchy (dough will be VERY sticky). If kneading by hand, this will take 10-15 minutes. Your dough will be ready when it passes the window test: pull a quarter-sized piece of dough between your hands and put it up to a window. If you can see the light shining through and the dough does not break, the glutens are strong enough and your dough is ready.
Grease a large bowl with oil and place dough into bowl. Cover with plastic wrap and allow to rise in a warm place for 1 1/2 hours.
for the pastry cream
Meanwhile, make your pastry cream. In a small saucepan over low heat, combine dried hibiscus, milk, 1/4 cup of the sugar, lemon juice, and lemon zest. Heat, stirring occasionally, until milk is scalding (steaming) but not boiling.
In a separate bowl, combine egg yolks, the remaining 1/4 cup of sugar, and flour. The mixture will be pasty and this is OK. Just make sure everything is well mixed together.
Pour 1/4 cup of the hot milk mixture into the egg mixture and whisk to combine, then slowly pour everything into the saucepan, whisking the entire time.
Turn the heat to medium and whisk slowly but constantly until the mixture thickens to a pudding-like consistency. Remove from heat and strain through a fine mesh sieve into a bowl.
Cover pastry cream with plastic wrap and place in fridge to cool completely.
back to the donuts
Punch dough lightly to deflate, then roll into a lightly floured work surface. Roll dough to 1/4 inch thick, then use a round 3-inch pastry cutter to cut dough into circles and place on a lined baking sheet. Cover loosely with a towel or plastic wrap and let rise for 1 hour.
Mix together remaining 1 cup of sugar with finely crushed dried hibiscus flowers. Set aside.
In a large heavy-bottomed pot, heat 2 quarts of canola oil to 350°F. Fry donuts 3-4 at a time for 30 seconds to 1 minute on each side, until lightly golden brown. Remove from heat and toss in hibiscus-sugar mixture.
Last, scoop pastry cream into a piping bag with a 1/2 inch tip. Pipe pastry cream into the center of each donut. DEVOUR!