Special Equipment: a doughnut cutter
For the doughnuts: Combine the yeast and 4 ounces warm water to activate the yeast. Using a blender or immersion blender, puree the milk and kale until smooth.
Place the flour in the mixing bowl of stand mixer fitted with a paddle attachment. Add the yeast mixture, kale puree, butter, sugar, salt and eggs to the flour. Knead for a couple of minutes.
Set the dough in a bowl and cover with plastic wrap or a clean dish towel, and set aside for 30 minutes.
Roll out the dough on floured surface. Using a doughnut cutter, cut the dough into doughnut shapes.
In a large, heavy-bottomed saucepan, pour enough oil to fill the pan about halfway. Heat over medium heat until a deep-frying thermometer inserted into the oil reaches 360 degrees F.
Fry the doughnuts for 2 minutes on each side, and then set on a cooling rack.
For the glaze: Combine the sugar, cream, maple syrup, vinegar and salt in a bowl, and whisk until smooth.
For the studs: Combine the bacon and pecans in a saute pan over medium heat and add the salt and maple syrup. Cook to just coat the bacon and pecans, 1 minute. Cool.
Dip the doughnuts in the glaze and stud the top with the bacon and pecan mixture. Add crispy fried kale leaves on top to garnish if using.