1. Adjust oven to upper-middle rack position, heat to 350 degrees
2. place cut butter in 13x9 inch baking dish and heat in oven until melted (8-10 min)
Meanwhile
3. Mix 1/4 cup sugar and lemon zest in food processor (about 5 pulses)
4. Mash blueberries and 1 tablespoon of lemon sugar in bowl
5. Melt remaining (8oz) butter.
6. Combine flour, remaining (1 1/4 cups) sugar, baking powder, and salt in large bowl. Whisk in milk and melted butter until smooth.
7. Remove buttered baking dish from oven to wire rack, pour batter into prepared dish.
8. Dollop mashed blueberry mixture evenly over batter, sprinkle with remaining lemon sugar.
9. Bake until golden brown (45-50 minutes), let sit for 30 minutes on wire rack to cool before serving.