Preheat the oven to 400˚F. Line a jelly roll pan with foil (or parchment paper) and grease well. Spread the saltine crackers on the foil in a single layer. Set aside.
Put the butter and brown sugar in a saucepan and bring to a boil over medium heat, stirring often. Boil for 3-4 minutes.
Remove from heat and CAREFULLY stir in the vanilla. The mixture will splatter while stirring so use caution and possibly wear oven mitts.
Pour the caramel mixture over the saltine crackers and spread out evenly.
Bake at 400˚F for 5-6 minutes.
Remove from oven and sprinkle with semi-sweet chocolate chips. Allow the chocolate chips to soften for about 5 minutes and then spread it out over the toffee.
Melt the white chocolate chips and shortening in the microwave on high for 45 seconds. Stir and continue heating in 10-second intervals, stirring each time, until smooth. Drizzle over the chocolate in horizontal lines. Then, drag a knife vertically through the white chocolate and semi-sweet chocolate, alternating directions, to create a pattern. (See the video for the technique.)
Sprinkle with chopped toasted pecans on 1/2 of the pan and add sprinkles to the other half.
Allow the toffee and chocolate topping to cool and harden before removing it from the pan. Peel the foil off before cutting (or breaking) the toffee into pieces. Or you can peel the foil off as you go if that is easier.
Video
Notes
Store in an airtight container in the fridge for up to a week or in the freezer for up to two months.