Step 1

Arrange a rack in the middle of the oven and heat the oven to 350°F. Line a rimmed baking sheet with aluminum foil.

Step 2

Arrange 4 sheets matzo on the baking sheet, breaking them up and fitting them together to form a single layer.

Step 3

Place 1 1/4 cups packed dark brown sugar and 2 sticks unsalted butter in a medium saucepan over medium-high heat and cook, whisking occasionally, until the butter melts and the mixture comes to a boil. Reduce the heat to medium and continue to cook, whisking constantly, until the mixture thickens, about 3 minutes.

Step 4

Remove from the heat and whisk in 1 tablespoon vanilla extract and 1 1/2 teaspoons kosher salt. Pour the mixture over the matzo, then spread into an even layer with an offset spatula.

Step 5

Bake until the caramel darkens and is bubbly, about 15 minutes.

Step 6

Remove the matzo bark from the oven and immediately sprinkle 1 to 1 1/2 cups chopped bittersweet chocolate or chocolate chips over the caramel. Let sit for 5 minutes for the chocolate to melt.

Step 7

Use the offset spatula to evenly spread the melted chocolate over the caramel. Sprinkle with the flaky sea salt. Let cool until the baking sheet is warm to the touch. Refrigerate until completely chilled, about 1 hour.

Step 8

Cut or break into squares and serve cold. You may serve the bark at room temperature as well but the chocolate can get a bit melty.