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 constantly until melted and smooth. Then remove from heat and whisk in 2 tablespoons the powdered sweetener until smooth.

Pour the peanut butter mixture into the prepared baking pan and spread evenly to the edges.

In a small saucepan over medium heat, bring the whipping cream to a simmer for 1 minute. Remove from the heat and add the chopped chocolate. Let sit for 5 minutes to melt.

Add the remaining 2 tablespoons of sweetener and the vanilla extract and whisk vigorously until smooth.

Drizzle the dark chocolate over the peanut butter mixture and swirl gently with a knife. Refrigerate for at least 2 hours until firm.

Lift the tiger butter out of the pan by the overhanging parchment and use a sharp knife to cut into 16 bars.