Grease and flour an 8x8-inch pan and preheat oven to 325 degrees F. Combine the sugar and mint in the bowl of a food processor and process for 30 to 60 seconds until the mint is pulverized. Set aside. Melt the butter in a 2-quart saucepan over low heat. Turn off the heat and add the chocolate. Set aside for 5 minutes to melt. Whisk chocolate to smooth, then stir in the mint sugar mixture. Whisk in the eggs and vanilla.
Mix together the flour and salt, then stir into the chocolate mixture until evenly blended. Fold in the mint chocolate chips. Scrape the batter into the prepared pan and bake for 35 minutes. When done, the surface will be quite soft. Cool overnight on a rack before slicing.