Melt chocolate in the microwave and set aside to cool slightly. In the bowl of an electric mixer, beat melted chocolate, butter, corn syrup, egg, and sugar on low speed until well incorporated. Sift together flour, salt, and baking soda. Slowly add dry ingredients to mixer running on low speed. Increase speed to mix until just combined. Chill dough in an airtight container for at least two hours or overnight.
Using a small cookie scoop or tablespoon measurer, make small balls of dough. Roll in sugar and place on a parchment-lined or ungreased baking sheet. Put in a preheated 350-degree oven for 10-12 minutes, until set but slightly soft in the middle of the cookie.
Let set on pan for two minutes. While warm, turn half of the cookies upside down. Place thin mint on top and immediately set another cookie on top, creating a sandwich. Let thin mint melt then twist and gently push cookies together. This spreads the thin mint to the edges and seals the "sandwich."