Step 1

Preheat oven to 350 F. Place 4 tablespoons cut-up butter in a 9x13-inch baking pan and put in the oven until butter is melted, 5-7 minutes. Continue with the rest of the instructions, removing the pan from the oven when the butter is melted but leaving the oven on.

Step 2

In a food processor, pulse 1/4 cup sugar and lemon zest in food processor until combined, about 5 pulses. Set aside. (If you don't have a food processor, combine the sugar and lemon zest in a small bowl and rub your fingers through the mixture until well combined.) Using a potato masher, in a medium bowl, mash the blueberries with 1 tablespoon of the lemon sugar until the berries are coarsely crushed.

Step 3

In a large bowl, combine flour, remaining sugar, baking powder and salt. Whisk in the milk and the 8 tablespoons melted butter until smooth. Pour batter into the baking dish with the melted butter. Dollop spoonfuls of the mashed blueberry mixture evenly over the batter. Sprinkle with the remaining lemon sugar and bake until golden brown and edges are crisp, 45 to 50 minutes. Cool 20-30 minutes before serving. Serve with a scoop of vanilla ice cream, if desired.