• Preheat oven to 375°F and line a 9x13-inch baking pan with parchment paper.

To make the butter crust:

• Beat butter in a stand mixer fitted with a paddle attachment on high speed for 1-2 minutes until smooth. Scrape down bowl and add sugar, then continue to beat until pale and creamy, another 2-3 minutes.
• Add egg yolks one at a time, mixing after each, then add vanilla.
• Add the flour and salt, and mix on low speed until combined into a soft dough.
• Press a generous ⅔ of the dough into the prepared pan, saving the remaining 1/3 for the top.

To make the filling:

• In a large bowl, toss the blueberries, sugar, cornstarch and lemon zest together. Spread into the even layer over the crust.
• Scatter pieces of the remaining dough over the top of the blueberries.
• Bake until topping is golden brown and filling is bubbling, about 45 minutes.
• Let cool for at least 30 minutes to finish setting up, though I prefer to let the bars cool completely. ThenI use the parchment paper as a sling to lift the bars out of the pan for easy slicing.
• Serve bars warm or at room temperature, with vanilla ice cream or whipped cream if desired.