Melt the margarine in a saucepan over low heat. You can also melt it in the microwave, but to avoid splatters, put the margarine in a bowl covered with plastic wrap with a few holes poked in it. Once melted, allow the margarine to cool for a few minutes.

Combine the oats, sugar, flour, and salt in a mixing bowl. Stir the melted, cooled margarine into the dry ingredients. Stir in the egg and vanilla extract.

Add the baking powder and baking soda and mix until thoroughly combined.

Cover the dough with plastic wrap and chill for at least 2 hours. Preheat the oven to 350°F. Line a baking sheet with parchment paper.

After the dough has rested, scoop out spoonfuls of dough and roll into balls. It’s best to weigh the balls of dough so you’re sure to have identical cookies to sandwich together. The size I find best weighs ⅓ ounce. Place the balls of dough on the prepared baking sheet, leaving 2 inches between them.

Bake for 8 to 9 minutes. The cookies should be golden around the edges with a lightly colored center.

Using a spatula, transfer the cookies to a rack and allow to cool completely. Repeat with the remaining dough. As soon as the cookies have cooled, place them in an airtight container—they can’t take any humidity at all! To make chocolate-sandwich havreflarns, melt the chocolate over a hot water bath or in 20-second bursts in the microwave oven, stirring after each time.

If you wish, you can temper the chocolate, but for this you’ll need couverture chocolate, which you can find online or at specialized stores. Follow the tempering curves indicated on the packaging. The addition of cacao butter will allow you to temper the chocolate without following the temperature curves. But there’s no need to complicate this step unduly: You can also use ordinary chocolate. The only risk is seeing a few white streaks when the chocolate sets.

Using a dipping fork, dip the bottom of each cookie in the melted chocolate, just enough to coat the rim; only the bottom and rim of the cookie should be coated in chocolate. Keeping the cookie on the fork, transfer it to a cooling rack, turning it upside down so that the chocolate side is on top. Take a second cookie and place it on top of the chocolate filling. Repeat this step with the remaining cookies.

Allow the chocolate to set and return the sandwich cookies to the airtight container. Well protected from humidity and air, these cookies keep for up to 1 month.