Step 1

Make the crust:

Step 2

Heat the oven to 350 F. Line a 9- by 13- by 2-inch baking dish with parchment paper, leaving an overhang.

Step 3

In the bowl of an electric mixer fitted with the paddle attachment, beat the soft butter and granulated sugar on high until light and fluffy, about 3 minutes.

Step 4

In a medium bowl, whisk together the flour and salt. With the mixer on low, slowly add the flour mixture. Increase the speed to high and beat until the dough holds together, about 1 minute more.

Step 5

Turn out the dough into the prepared pan and press it into an even layer using your fingertips.

Step 6

Bake until the edges are just beginning to brown, 18 to 22 minutes.

Step 7

Make the filling:

Step 8

In a medium bowl, whisk together the brown sugar, melted butter, eggs, vanilla, apple cider vinegar, flour, baking powder and salt.

Step 9

To assemble:

Step 10

Pour the filling into the hot crust and continue baking until the crust is golden and the edges of the filling are just set (the center will be slightly jiggly), about 25 minutes more.

Step 11

Cool in the pan for 1 hour (the filling will continue to set as it cools), then remove from the pan using the parchment. Cut the bars into 12 even squares for serving.

Step 12

Bars can be stored in an airtight container in the fridge for up to one week.