Heat oven to 350°F. Line a 9×13-inch pan with foil and lightly grease.
Arrange graham cracker sheets in a single tight layer to fully cover the pan.
Melt butter and brown sugar in a saucepan over medium heat, stirring until smooth.
Bring the mixture to a steady simmer and cook, stirring, for about 3 minutes until glossy and thickened.
Remove from heat. Stir in vanilla and kosher salt, then immediately pour toffee over the crackers. Spread evenly.
Bake until bubbling over the entire surface, 8–10 minutes.
Scatter chocolate chips over the hot toffee. Let sit 2 minutes, then spread into a smooth layer.
Sprinkle pecans over the chocolate and finish with flaky sea salt, if using. Press gently to adhere.
Cool to room temperature. Chill 20–30 minutes to set, then lift out and cut into bars.