Preheat the oven to 350 (F). Line a baking sheet with parchment paper or a silicone mat.
Whisk together 1/2 teaspoon salt, 1 cup brown sugar, and 1 cup corn syrup until the mixture is smooth and consistent.
Use a rubber spatula to stir in 1 cup flour, 3/4 cup melted butter, 1/2 teaspoon vanilla extract, and 1 cup oats. At this point, the batter will be very sticky and hard to stir -- but persevere! It'll be a good workout for your arms!
Drop the batter onto the prepared sheet pan using a 1-teaspoon measuring spoon. Leave plenty of room since the cookies will spread a great deal. I know one teaspoon of batter doesn't sound like much, but believe me -- these babies will spread! Each cookie eventually ended up being something like 4 inches wide. I found five cookies per pan (sized 18" x 13") to be ideal. Position the batter so that it looks like the 5-side of a dice, if you know what I mean.
Bake for 8 minutes or until the edges are brown. When ready, leave the cookies to cool flat on the sheet pan.
Repeat above steps until the batter is finished. Since you have a lot of batter and can only bake a few cookies at a time, I found the best way to do this is to actually prepare 2 baking sheets, alternating baking them. By the time the second baking sheet of cookies has finished baking in the oven, the cookies from the first baking sheet will have cooled. Transfer those cooled cookies onto a flat surface to finish cooling and prepare this baking sheet with batter to replace the baking sheet in the oven.
Once the batter is finished and you have a ton of cookies that have cooled/are cooling, melt 8 ounces dark chocolate in a double boiler. Watch out -- chocolate burns easily, so be sure you're stirring the mixture constantly.
When the chocolate has melted completely, spread a small amount of chocolate on the back of half of the cookies and lay another cookie on top to make a sandwich. Leave to cool and set.