Line a square 8-inch baking dish with wax paper so that you have a wax paper overhanging on both sides. This will allow for easy removal later on.
Make the bottom layer:
Melt the butter in a medium microwave-safe bowl. Allow it to slightly cool.
Mix in the stevia, peanut butter, and almond flour. Start mixing with a rubber spatula, then use a hand whisk to mix thoroughly until completely smooth and no almond flour lumps remain.
Use the spatula to transfer the mixture to the prepared pan, and to spread the mixture evenly. Freeze for 15 minutes.
Make the top layer:
Place the chocolate chips and the butter in a microwave-safe bowl. Microwave in 30-second increments, stirring after each microwave session, until almost fully melted. Stir the mixture with a rubber spatula until fully melted.
Pour the melted chocolate on top of the peanut butter layer, using the spatula to scrape it off the bowl and to spread it evenly. Work gently, so that the chocolate layer stays on top of the peanut butter layer.
Cover the pan with plastic wrap. Refrigerate until set and firm, about 4 hours.
Carefully lift the wax paper and place it on top of a cutting board. Using a sharp chef’s knife, cut into 16 squares.
Keep leftovers refrigerated in an airtight container, separated by layers of wax paper.
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Notes
It's best to use a freshly opened jar of peanut butter, since the bottom of these jars is often a bit too dry, even if you stir the peanut butter well when you open them.