Line a 9x11-inch rimmed sheet pan with parchment paper. Set aside.
Cook butter and sugar in a large saucepan over medium-high heat, stirring frequently. When butter has melted and sugar has dissolved, cook for 10 minutes, or until color turns from pale yellow to light brown.
Use a candy thermometer to ensure that the butter/sugar mixture reaches a temperature of at least 290°F. (soft crack stage), and preferably 300℉ (hard crack stage). If the mixture isn't cooked long enough, the consistency of the candy will be too soft, more like caramels than toffee.
Stir in vanilla extract and salt, then immediately pour the mixture into the parchment lined pan.
Scatter a layer of chocolate chips evenly over the hot toffee. Wait 5 minutes, then use a heat-proof spatula to spread the chocolate into a smooth layer.
Garnish with Oreos, chocolate chips, M&M’s, pretzels, and sprinkles.
Refrigerate for 1 hour, then break into pieces.
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Notes
Be very careful working with the hot candy mixture, as it will burn your skin quickly if it makes contact. Keep all distractions out of the kitchen, and be sure young kids and pets are at a distance.
Test the accuracy of your candy thermometer by placing it in a pan of boiling water. The thermometer should read 212º Fahrenheit when the water is at a full boil.
Make sure the tip of the thermometer does not rest on the bottom of the pan. Otherwise, you will not have an accurate temperature of the sugar mixture.
Work quickly: When you pour the hot toffee mixture into the pan, be quick about getting the chocolate chips scattered over it, so they can melt properly. After spreading the melted chocolate over the toffee, quickly add toppings because the chocolate will begin to cool quickly.
Variations - You can use other flavors of chocolate chips if you prefer, such as peanut butter, butterscotch, white chocolate, etc. Other fun toppings are various cookies, crumbled pop tarts, crumbled bacon, more toffee pieces, mini chocolate chips, etc.