In a medium-size bowl using a fork mix all the ingredients together until the mixture forms small bits and pea size chunks; set aside.
Directions for muffins
In a large bowl whisk together the flour, baking powder, baking soda, and salt.
In a medium-size bowl lightly beat the eggs, and then mix in the melted butter, oil, vanilla, sugar, and buttermilk; mix until well combined.
Mix the wet ingredients into the dry ingredients until just moistened and then fold in the blueberries.
Preheat oven to 400F.
Spray a 6 cup jumbo muffin tin with cooking spray including the top of the tin. Using a heaping ½ cup measuring cup fill the muffin tins with the batter. (Cups will be very full) sprinkle the Streusel over the top of the muffins.
Bake for 25 to 30 minutes until muffins are golden brown on top and a table knife inserted in the center comes out clean.
Let muffins cool for about 5 minutes, remove them from the muffin tin, and cool on a wire rack for 5 more minutes – serve hot with butter.