Line a large sheet pan with parchment paper and set aside.
In a medium saucepan over low heat, combine butter and brown sugar. Toss in a large pinch of salt. Stir constantly with a heat-proof spatula for 10-15 minutes. Your toffee should slowly become golden brown in color. You’ll know your toffee is ready when it is a dark golden brown color and reaches 300°F (use a candy thermometer to check!).
Pour the toffee onto the sheet pan. Working fast, use your spatula to evenly distribute the toffee around the pan. You should aim for a thickness of a ¼-1/3 of an inch. Allow to cool for 3-5 minutes so the toffee can set slightly.
Sprinkle your chopped chocolate over the toffee. Allow the chocolate to melt for 2-3 minutes, then using a spatula or the back of a spoon, smooth out the now melted chocolate. If lumps remain, give it another 2-3 minutes, then smooth out again until the chocolate is completely melted.
Last, melt the white chocolate in a microwave of 50% heat for 15 seconds. Stir. If not fully melted, give it another 15 seconds.
Scoop your white chocolate into a small ziplock container, creating a make-shift piping bag. Cut a TINY hole into one corner, then pipe spiderwebs all over the bark. Begin by creating four intersecting lines (like a star), then pipe U-shaped lines in between each line in 2-3 concentric circles (see bark for detail).
Top with raisins and colored sugar. Allow to cool completely before serving.*
*You can place this in the fridge for up to 20 minutes to help the chocolate harden. Any more and it will develop sweat stains. Gross.