In a large bowl, whisk together the flour, sugar, flaxseed, baking soda, powder, cinnamon and salt. In a separate bowl, whisk together the egg, yogurt, milk, butter and vanilla extract. Add the wet ingredients to the dry, stirring until just combined. Do not overmix! Use a spatula to stir in the blueberries. Fill the muffin tins 3/4 of the way full. Sprinkle the top of each muffin with the crumb mixture. Bake for 18-20 minutes, or until just set and golden on top. Let cool slightly before eating.