Dough: In a small bowl, mix together the 1/3 cup milk and vinegar, and set aside (around 5 minutes to “sour”)
In a large bowl, combine the flour, berry sugar, yeast and lemon zest.
In a medium bowl, whisk together the milk, eggs, butter and “soured” milk.
Slowly pour the wet ingredients into the dry ingredients, and knead until smooth and elastic. Cover, and let stand in a warm place until doubled in size.
Lemon Filling: In a medium saucepan, combine the sugar, cornstarch, salt and water on medium heat. Stir constantly until mixture thickens and boils. Continue to cook at a boil for 1 minute. Add in the lemon juice.
Remove from heat and add the lemon rind, butter and food coloring. Allow to cool before using.
Preheat the oven to 375˚F.
On a lightly floured surface, roll the dough to 5mm thick. Cut 48 rounds with a round cutter. Reroll the scraps if necessary. Place a heaped teaspoon of the lemon filling in the center of 24 of the round pieces. Brush the edges with milk and cover with the remaining rounds. Press to seal the edges well.
Transfer to a lightly sprayed baking sheet, then bake until lightly golden, around 12 minutes.
Meanwhile, melt the butter. In a small bowl, combine the sugar and lemon zest.
When the doughnuts are done, dip them in the butter and toss in the lemon sugar.