Step 1

Prepare 1 1/2 pounds fresh berries and place them in a large saucepan: trim and cut strawberries into 1/2-inch thick slices, halve blackberries, and leave blueberries whole.

Step 2

Finely grate the zest of 1 medium lemon (about 1 tablespoon) into the saucepan. Juice the lemon and add 2 tablespoons of the juice into the saucepan. Add 1/2 cup granulated sugar, 1 teaspoon vanilla extract, and 1/2 teaspoon kosher salt to the saucepan.

Step 3

Cook over medium-high heat, stirring occasionally, until the juices release and come to a boil. Boil for 1 minute. Reduce the heat to medium-low and simmer, stirring occasionally, until the juices reduce slightly and the berries soften, about 5 minutes. Meanwhile, whisk 2 tablespoons cornstarch and 2 tablespoons cold water together in a small bowl until the cornstarch is dissolved.

Step 4

While stirring constantly, add the cornstarch mixture and cook until the juices thicken, about 1 minute. Remove from the heat and let cool to room temperature, about 30 minutes. Meanwhile, prepare the crust.

Step 5

Arrange a rack in the middle of the oven and heat the oven to 350°F. Lightly coat a 9x13-inch baking pan with cooking spray. Line the pan with overlapping pieces of parchment paper so that there is excess parchment hanging off all four sides to create a sling, then coat the parchment with more cooking spray. (Alternatively, line with aluminum foil and then coat the foil in cooking spray.)

Step 6

Cut 2 1/2 sticks cold unsalted butter into 1/2-inch cubes. Beat 1 large egg in a small bowl until broken up.

Step 7

Option 1: Making the crust by handPlace 3 cups all-purpose flour, 3/4 cup granulated sugar, 1/4 cup packed light brown sugar, 1 teaspoon baking powder, and 1 teaspoon kosher salt in a large bowl and whisk to combine. Scatter the butter cubes over the flour mixture and drizzle with the egg. Using a pastry cutter or 2 small knives, cut the mixture together until the dough is crumbly, forms pea-sized crumbs, and holds together when pressed.Option 2: Using a food processorPlace 3 cups all-purpose flour, 3/4 cup granulated sugar, 1/4 cup packed light brown sugar, 1 teaspoon baking powder, and 1 teaspoon kosher in a food processor and pulse until combined. Scatter the butter cubes over the flour mixture and pulse until the butter pieces are no larger than a pea. Add the beaten egg and pulse until the mixture holds together when squeezed. Transfer to a large bowl.

Step 8

Transfer 2/3 of the dough (about 4 1/2 cups) to the baking pan and press evenly into the bottom with your hands or with the bottom of a measuring cup. Using your hands, squeeze the remaining dough in the bowl together to form crumbles no larger than a marble. If the filling is not cooled yet, refrigerate the crust and dough clumps.

Step 9

Transfer the berry filling onto the crust and spread into an even layer. Sprinkle evenly with the dough crumbles. Bake until the top is golden brown and the filling is bubbling, about 50 minutes. Place the pan on a wire rack and let cool completely, about 3 hours.

Step 10

Place 1 cup powdered sugar, 2 tablespoons milk, 1 teaspoon vanilla extract, and 1/4 teaspoon kosher salt in a small bowl and whisk until smooth.

Step 11

Remove the crumb bar slab from the pan by grasping the parchment or foil sling. Place on a cutting board. Drizzle with the glaze. Let sit for 30 minutes for the glaze to set. Cut into 24 bars. Serve topped with ice cream if desired.