Preheat the oven to 375 degrees. Spray the muffin plan with cooking spray or place muffin liners in the pan.
Melt the butter in a small saucepan over medium low heat. Stick close to the stove. Once all of the water has cooked out of the butter, it will brown quickly. When it is a medium brown color, remove it from the stove and pour it into another bowl to stop the cooking. Cool slightly.
Add milk, egg, yolk and vanilla to a medium bowl and whisk together. Slowly pour in the cooled brown butter and whisk again until it is combined.
In another medium bowl, add 1 cup all-purpose flour, ½ cup whole wheat flour, ¾ cup sugar, baking powder and salt. Whisk to combine. Pour the milk mixture into the flour mixture all at once and stir gently to combine. Carefully stir in the lemon zest, poppy seeds and blueberries. Divide batter among muffin cups.
Combine 3 tablespoons diced butter, ½ cup whole wheat flour and 3 tablespoons sugar in a small bow. Use two knives or a pastry blender to cut the butter into the flour and sugar. It should resemble small peas. Sprinkle topping evenly over muffins.
Bake for 18 - 20 minutes until tops are golden and a skewer inserted into the center of the muffins comes out clean. Cool pan on a wire rack for 15 minutes. Remove muffins from pan and continue to cool on a wire rack.
If using frozen berries, do not defrost them before tossing them in the batter.barely adapted from Joy the Baker