Instructions
Step 1
Position a rack in the center of the oven and preheat to 350°F. Butter and flour a 9-by-13-inch metal baking pan. Using a stand or handheld mixer on medium-high speed, beat 16 tablespoons of the butter and ½ cup of the sugar until pale and fluffy, about 2 minutes. Add 2½ cups of the flour and ½ teaspoon of the salt and beat until just combined. Press the dough (see headnote) evenly into the bottom and halfway up the sides of the prepared pan. Chill for 20 minutes, then bake until lightly browned, about 20 minutes. Set aside to cool completely.
Step 2
Meanwhile, make the streusel: In a small bowl, use a fork to combine ¼ cup of the sugar; the vanilla; and remaining butter, flour, and salt until coarse crumbs form.
Step 3
In a medium pot set over medium-high heat, stir together 3 cups of the berries, the lemon juice, cinnamon, and remaining ¾ cup of sugar. Cook, stirring occasionally, until the berries burst, about 20 minutes. Remove from the heat and stir in the remaining berries. Set aside.
Step 4
In a small bowl, stir together the poppy seeds and cream, then use a silicone spatula to spread evenly over the crust. Top with the blueberry mixture, spreading it to the edges, then sprinkle with the streusel. Bake until bubbly and lightly browned, about 40 minutes. Allow to cool completely before cutting into squares.