How to Make It
In a small saucepan, warm the milk over moderately low heat to 110°. Pour into a large bowl and stir in the yeast and 1 tablespoon of the sugar. Let stand until foamy, 10 minutes. Whisk in the sour cream, egg, vanilla, salt and the remaining 1 tablespoon of sugar. Add all but 2 tablespoons of the flour and stir until a shaggy dough forms.
Coat a large bowl with nonstick cooking spray. Scrape the dough out onto a lightly floured surface and knead until very smooth, 6 minutes. Form into a ball and transfer to the bowl. Cover tightly with plastic wrap and let stand in a warm place until doubled in bulk, 1 1/2 hours.
In a bowl, blend the butter with the remaining 2 tablespoons of flour until smooth. Scrape the butter onto a large sheet of plastic wrap, shape it into a 6-inch square and wrap well. Refrigerate until barely firm, about 15 minutes.
On a lightly floured surface, roll out the dough to a 10-by-16-inch rectangle. Set the butter square in the center of the dough. Fold the short sides of the dough over the butter to enclose it; pinch the open ends of the packet to seal. Rotate the packet so that one pinched end is facing you. Roll out the dough to a 15-by-8-inch rectangle. (The butter should be pliable; chill the dough for 10 minutes if the butter is too soft.) Fold one-third of the dough into the center and the other third on top, like you would fold a letter. Turn the dough 90°. Roll out the dough again to a 15-by-8-inch rectangle and fold like a letter again. (This is 2 turns.) Wrap in plastic and refrigerate for 1 hour. Repeat the rolling, folding and chilling 2 more times for a total of 6 turns.
Preheat the oven to 350°. On a rimmed baking sheet, toast the nuts for 12 minutes, until golden. Let cool, then finely chop. In a bowl, mix the nuts with the flour and brown sugar. Mix in the maple syrup, 1 stick of butter, vanilla and salt. Cover and refrigerate for 10 minutes. Leave the oven on.
On a lightly floured surface, roll out the dough to a 30-by-8-inch rectangle. Spread the filling down the length of the rectangle, leaving a 2-inch border of dough on each side. Fold one long side over the filling, then fold the other long side on top, overlapping by 1/2 inch; pinch to seal. Slide the dough onto a large sheet of parchment paper and roll it over so it’s seam side down. Shape the dough into a ring: Moisten the inside of one end with water and place the other end inside, pinching to seal. Slide the parchment and kringle onto a rimmed baking sheet. Refrigerate until chilled, 30 minutes.
Brush the kringle all over with melted butter and sprinkle with turbinado sugar. Bake the kringle in the center of the oven for about 50 minutes, until puffed and golden; some of the filling may seep out. Let the kringle cool for 30 minutes.
In a medium bowl, whisk all of the ingredients together until smooth. Drizzle the glaze over the kringle and let stand for 15 minutes before cutting into wedges and serving.