• Warm kefir (or milk) to 110F. Combine the warm kefir (or milk), active try yeast and 1 teaspoon on sugar in a small bowl. Give it a gentle stir and set aside until it activates (doubles in size).
• In the bowl of a stand mixer fitted with the hook attachment, combine the flour, sugar, salt, yeast mixture, butter and eggs, and mix on low speed for 5-6 minutes until dough comes together (it should come off the sides of the bowl).
• Lightly flour a working surface and roll out the dough to about 1/2 inch thick. Using a 3 inch biscuit or round cookie cutter, cut out around 12-14 donuts.
• Transfer to a baking sheet with parchment paper and sprinkle some flour over the top. Cover with a kitchen towel and allow to rise for 1.5 hours.
• In a heavy duty pan, heat the vegetable oil to 350F (using a candy thermometer), fry 3 donuts at a time until golden brown (each side takes about 1 minute) on both sides.
• Drain them on paper towel and toss them in the cinnamon sugar mixture.
• Using a knife, cut a slit at the top and fill with dulce de leche (using a piping bag with a large star tip).

Churro Topping:

• In a medium bowl, whisk the sugar and cinnamon until combined. Set aside.

Filling:

• Using a spoon, stir the dulce de leche before transferring to a piping bag fitted with a large star tip.