Line the pan: heat the oven to 400°F (200°C). Line a 10x15-inch rimmed baking sheet with foil, then spray the foil lightly with cooking spray. Arrange the saltine crackers in a single layer, edges touching, trimming the last row to fit.
Make the toffee: in a medium saucepan, melt the butter with the brown sugar and salt over medium heat, stirring until the sugar dissolves. Bring to a full boil and boil for exactly 3 minutes without stirring, until thickened and slightly darker.
Add the vanilla: remove the pan from the heat and stir in the vanilla extract. The toffee will bubble up — that is normal.
Pour and spread: pour the hot toffee evenly over the crackers and spread quickly with an offset spatula or the back of a spoon so every cracker is covered edge to edge.
Bake: bake for 5 to 6 minutes, until the toffee is bubbling all over. Watch closely — it goes from perfect to burnt fast.
Add the chocolate: sprinkle the chocolate chips evenly over the hot toffee, then return the pan to the oven for 1 minute. Remove and spread the melted chocolate into a smooth, even layer.
Top it: while the chocolate is still wet, scatter the pecans and sprinkles over the top and add a pinch of flaky sea salt if you like. Press the toppings in very lightly.
Chill and crack: let the pan cool for 10 minutes, then refrigerate for at least 1 hour until completely firm. Lift the bark out by the foil and break it into pieces with your hands.