Line an 8-inch square baking pan with parchment paper, allowing paper to come up the sides for easy removal.
In a medium saucepan over medium heat, combine the peanut butter, white chocolate chips, and cocoa butter. Whisk until melted and smooth. Then whisk in 2 tablespoon of the powdered sweetener until smooth.
Pour the peanut butter mixture into the prepared baking pan and spread to the edges.
In a small saucepan over medium heat, bring the whipping cream to a simmer. Remove from the heat and add the chopped chocolate. Let sit a few minutes to melt.
Whisk until smooth, then whisk in the remaining 2 tablespoon of powdered sweetener and the vanilla extract.
Drizzle the dark chocolate over the peanut butter mixture and swirl with a knife. Refrigerate until firm.
Lift the tiger butter out of the pan by the overhanging parchment and use a sharp knife to cut into 16 bars.