1For the struesel: In a medium bowl, combine all of the ingredients. Using clean hands, crumble it all together. Refrigerate until ready to use.
2For the muffins: Make the buttermilk mixture by pouring milk atop vinegar and allowing this to rest.
3Make the flax "egg" by combining the milled flaxseed with water.
4In a large mixing bowl, combine the whole-wheat flour, sugar, baking soda, baking powder and salt.
5In a smaller bowl, combine the buttermilk, flax egg and vanilla.
6Fold in the raspberries. (This coating of flour prevents them from sinking to the bottom of the muffins, but may tint the muffins. Fold in the raspberries at the end if you don't want to muffins to be slightly pink.)
7Pour the buttermilk mixture into the flour mixture in three sections, gently stirring after each new addition.
8Either grease a cupcake pan or line it with cupcake liners. Fill each slot two-thirds way with batter. Top each with ~1 T of the struesel topping.
9Bake in a 350 degree oven for 20-25 minutes, until firm, lightly browned and a wooden skewer comes out clean.