Preheat oven to 400 degrees. Line one 6-cup jumbo muffin pan with paper liners.

In a small bowl, combine the coarse sugar, 1/3 cup flour and cold butter. Using a pastry cutter, mix the topping well, until it is fine and crumbly. Store in refrigerator while making muffin batter.

In a large bowl, whisk together the 1 and 1/2 cups of flour, sugar, baking powder and salt. In a 1-cup glass measuring cup, measure the oil and add the egg. Whisk together. Pour in enough buttermilk until liquid is 8 fluid ounces... a little more than 1/3 cup of buttermilk. Add the vanilla and whisk again.

Pour the wet mixture into the dry mixture and stir gently with a wooden spoon or spatula until just a few streaks of flour remain. Add the blueberries and mix gently until evenly distributed in the muffin batter.

Divide batter evenly between 6 muffin cups (about 1/2 cup batter each). Sprinkle the top of each muffin generously with the topping.

Bake muffins on the center rack of oven for 28 - 30 minutes, or until toothpick inserted in center of muffin comes out clean. Allow to cool completely. Store in an air-tight container for up to 2 days.