Preheat oven to 350F. Line a 9 x 13 pan or similar with aluminum foil and spray it very well with cooking spray. Do not try to make this without using foil; you will hate yourself.
Place saltine crackers in a single layer on the bottom of the pan. Some recipes suggest using a jelly roll pan which may allow for a slightly bigger batch, i.e. 40 crackers, but it’s a bit too shallow for my comfort zone and didn’t want any bubble-overs.
In a saucepan on the stovetop, combine butter and sugar and bring to a boil while stirring constantly.
Once a boil is reached, reduce the heat and simmer, uncovered, for 3 to 5 minutes or until mixture has thickened some. Take care not that your simmer is not too fast/too high because it will be prone to bubbling over or scorching. Stir mixture frequently while it simmers to avoid bubbling over or scorching. The taste of burnt butter and sugar is awful so don’t burn it.
After mixture has thickened a bit, remove it from the heat, wait 30 seconds, add the vanilla extract and stir.
Pour mixture over the prepared pan with the saltine crackers.
Bake in for 5- 7 minutes, or until toffee/liquid becomes bubbly.
After removing the pan from the oven, let it rest for 3 to 5 minutes.
Sprinkle on the chocolate chips and wait for 3 to 5 minutes, letting them soften and melt a bit, and then spread and smooth them into an even layer with a spatula. * (see note below)
Let cool very well and if desired for expediting purposes, refrigerate or freeze until hardened. Break into pieces the size of your choice.
Store in an airtight container at room temperature for up to 2 weeks or in the freezer for up to 4 months.