Set the tart tin on a cookie sheet or flat tray so the removable bottom won’t come out of the pan while you are working with it.
Melt half the chocolate in the microwave in short bursts, stirring until the chocolate is melted and smooth.
Pour all but two tablespoons of the melted chocolate into the tart tin and spread it around to coat the entire inside of the tin. Use a spoon or brush to smooth chocolate up the sides of the tin, until the entire surface is covered.
Place the tart tin on a tray or baking sheet and put it in the freezer on a flat surface. When the chocolate has begun to set (about 5 minutes), use the reserved melted chocolate to reinforce the sides of the tart shell and fill in any thin parts or holes.
Return the tart shell to the freezer to set.
Make the peanut butter filling by beating the softened butter with the powdered erythritol with an electric mixer.
Add the Keto Chow and mix until combined. Add the heavy cream and continue to mix until the cream is absorbed. The mixture will be thick and pliable with a dough-like texture.
Pat the filling into a flat disc to fit into the chocolate tart shell.
Pull the chocolate out of the freezer and press the peanut butter filling into the tart pan, pressing it into the chocolate to fill the edges.
In a clean bowl, melt the remaining 8 oz. chocolate in the microwave using the same method as before.
Pour the melted chocolate over the peanut butter filling, smoothing it to the edges to be sure it meets the set chocolate and seals the filling in.
Sprinkle flaky sea salt, crushed peanuts, or mini chips over top while the chocolate is still warm, if desired.
Place the tart tin back in the freezer to set the chocolate.
When the chocolate has set, carefully remove the peanut butter cup from the tin by popping the bottom out gently.
Cut into pieces and enjoy.*