Position rack in center of oven. Preheat oven to 325°F. Line a 13x9-inch pan with aluminum foil or parchment paper extending over the short sides. Coat with nonstick spray; set aside.
Melt the butter in a large saucepan or microwave safe bowl. Whisk in sugar, then stir in flour and salt. Pat into even layer in prepared pan.
Bake for about 20 to 25 minutes or until just crust is very light brown all over, firm and dry. Cool on rack.
Melt chocolates and chocolate coating (is using) in separate bowls either in microwave or over water bath. Whisk peanut butter into melted milk chocolate.
Spread milk chocolate/peanut butter mixture evenly over cookie base. Place other chocolate colors/flavors in parchment cones and snip tip off of cone. One by one drizzle chocolates in random squiggles all over the milk chocolate. While chocolate is still wet, use a bamboo skewer or the tip of a small sharp paring knife to gently swirl and “marbleize” the chocolates. There is no right or wrong here; just swirl until you like the way it looks.
Chill pan until chocolate is set. Pull up on aluminum foil to release bars. Place on work surface, peel back foil and then cut cookies as you like. We cut random shapes so that they look like “marble slabs”. Store cookies in airtight container in single layers separated by parchment for up to one week. Let come to room temperature before serving, although we like eating them cold, too.
Recipe by Bakepedia at https://www.bakepedia.com/marbleized-chocolate-peanut-butter-shortbread/