Preheat oven to 450 F. Line two large cookie sheets with parchment paper, or spray each with cooking oil.
Measure all dry ingredients for dough into a large bowl. Mix together with a pastry blender or wooden spoon. Add cold margarine and cut in with a pastry blender (or two knives) until the margarine is in barely visible pieces. Make a well in the center of flour mixture, and pour in cold milk. Stir well to blend, then dump contents of bowl onto a well floured pastry mat or counter top. Gently knead 5 or 6 times until dough is no longer sticky and can be patted into a ball. Pat down dough, then roll out dough to a large 10" to 12" x 16" to 18" rectangle. Let rest five minutes or so while preparing filling.
For the filling, melt 1/4 c margarine; add brown sugar and cinnamon, mixing well. Spread cinnamon mixture evenly over dough. Sprinkle with blueberries, patting blueberries gently into the dough. Roll up dough from the bottom of the 18" side toward the top, pressing gently to maintain a tight roll. Completed roll will look like a long dough log. Cut dough into pinwheel slices approximately 1' thick and place each pinwheel on a parchment lined cookie sheet; place 8-10 pinwheels on each cookie sheet spacing out as much as possible.
Bake 450 F for 12 - 15 minutes, until lightly browned. While baking, create vanilla glaze if desired. To make glaze, add milk and vanilla to powdered sugar, and beat with a whip until a thin, smooth consistency is attained. Drizzle glaze in a zigzag pattern over cooled buns with a spoon.
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