Preheat oven to 350F and prepare a muffin pan with oil or muffin liners. In a large mixing bowl, cream butter and sugar together. Add eggs, mixing well to combine.
Add whole wheat flour, all purpose flour, salt and baking powder then mix until just combined. All milk and mix until just combined, being mindful not to over-mix!
Fold lemon zest and blueberries into the muffin batter, then divide batter into the muffin pan.
Whisk together brown sugar and cinnamon, then sprinkle approx. 1 heaping tsp of the cinnamon sugar on to each muffin.
Bake for 20-25 minutes until golden and a toothpick comes out clean. Cool muffins in the pan for 5-10 minutes, then transfer to a wire rack and let cool completely.
Store muffins at room temperature for up to 4 days or in the freezer up to 3 months.