| In a large bowl, stir together the yeast, water and 1 tbsp sugar. Let stand for about 10 minutes. Meanwhile, loosely wrap the butter in a large sheet of plastic wrap. Beat the butter, using a rolling pin, until the butter is malleable but still cold, set aside. Add the remaining 1/4 cup sugar, flour, salt, egg yolks, sour cream and zest to the yeast mixture. With an electric mixer, mix on low speed to blend all ingredients. Gradually add the butter and beat to form a soft, but not sticky, dough that just barely leaves the sides of the bowl. Beat or knead until very smooth and elastic, about 6 to 8 minutes. Transfer to an oiled bowl, turn dough to coat all sides of dough with oil. Lay a sheet of plastic wrap on top and refrigerate overnight. In food processor, combine the flour, cinnamon, and sugar. Add the vanillaand pulse to mix. Add the butter, a few pieces at a time, and pulse until mixture crumbles into pieces about twice the size of peas. Refrigerate or freeze the streusel if not using immediately. Line the bottom of a jelly roll pan with parchment paper. Butter the paper and the sides of the pan. Roll out the dough on a lightly floured surface to the size of the pan. Fit the dough in the pan. With sharp knife, cut the dough into 20 rectangles. Cover with a sheet of plastic wrap and let rise in a warm place until halfway risen or about 1 1/2 times its volume. Scatter about 1/3 of the streusel mixture over the half-risen dough. Cover again and let rise until fully doubled in volume. When fully risen, scatter the remaining streusel over the dough. Preheat the oven to 350 degrees. Bake until top is lightly browned and the edges are golden; about 30 to 35 minutes. Let cool briefly in the pan on a wire rack. Lightly dust with powdered sugar. Serve immediately. | 
