Heat the oven to 400°F (205°C). Line a rimmed 10×15-inch (25×38 cm) baking sheet with foil and spray with cooking spray.
Arrange the saltine crackers in a single layer, salty side up, covering the pan edge to edge.
Melt the butter and brown sugar in a saucepan over medium heat. Bring to a rolling boil and boil for 3 minutes, stirring constantly.
Pour the toffee evenly over the crackers and spread to the edges.
Bake for 5–6 minutes, until bubbling all over.
Remove from the oven and sprinkle the chocolate chips over the top. Let sit 5 minutes to melt, then spread into an even layer.
Sprinkle with the pecans and a pinch of flaky sea salt if you like. Cool 10 minutes, then refrigerate at least 1 hour until set.
Break into pieces and serve or package for gifts.