Preheat oven to 350F. Line 2 muffin pans (see note 1) with muffin liners and set aside
In a large bowl, sift 2 ⅓ cups flour, 1 cup sugar, 1 tsp baking powder, ½ tsp cinnamon and ½ tsp salt. Whisk in 2 tbsp flaxseed until combined. Set aside.
In another bowl, using a handheld electric mixer or a stand mixer with the paddle attachment, beat ½ cup melted butter, 2 eggs, 1 cup yogurt, 2 tbsp milk and 1 tsp vanilla on medium speed until smooth and incorporated (2-3 minutes).
Fold your wet ingredients into your dry ingredients just until combined. Add white chocolate and blackberries and stir just until evenly distributed. Do not over mix. The batter will be thick and lumpy.
Scoop the batter into your muffin liners (at least ¾ full), making sure you get a piece of fruit in every scoop.
Bake 30-35 minutes or until golden brown and the muffins bounce back when gently pressed. Cool in the pan for 5-10 minutes before transferring to a wire rack to cool completely.
This recipe yields 14-16 muffins depending on how much batter you put in your muffin cups.