Whisk sugar, yolks, and cornstarch in a medium saucepan until smooth. Add butter, TAZO Chai Concentrate, and milk; bring to a boil. Reduce heat to medium; cook, stirring, until custard is thick, 3 to 5 minutes. Pour through a mesh strainer into a bowl; cover surface with plastic wrap. Chill completely.
In a small mixing bowl place the yeast, 60 ml warm water and a pinch of sugar and let stand for 3-5 minutes until foams.
In a big mixing bowl place 315 grams flour, create a hole in the middle of the pile and add the yeast mixture mixing slowly. Add the warm milk, yogurt, Tazo® Concentrate Classic Chai Latte, sugar and vanilla extract and mix all well (can be done in a food processor) than add the salt and the lemon zest and mix it in.
Add the yolks and the margarine and the rest of the flour (60 gram)as needed so dough would not be too sticky but not too dry as well. Transfer to an oiled, big mixing bowl and let rise for an hour and a half, until doubles in volume.
Dip fry the donuts in vegetable oil heat to 190°C, turning the donuts so they are golden from all sides. Remove to a tray lined with paper towels. Allow to cool.
Make small hole in donut and pipe in Tazo Chai Pastry Cream and dust with confectioners sugar.