In a large saucepan over low heat, melt the butter with the brown sugar and water, stirring until smooth.
Add the semisweet chocolate chips to the butter mixture and stir until they are partially melted, then remove from heat and continue stirring until fully melted. Let cool for about 10 minutes.
At high speed, beat the eggs into the cooled chocolate mixture one at a time until incorporated.
Reduce mixer speed to low and add the flour, baking soda, and salt; beat until just blended into a dough.
Cover the dough and chill in the refrigerator for about 1 hour.
Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper or leave them ungreased.
Using a tablespoon or 1-tablespoon cookie scoop, roll the dough into balls and place them about 2 inches apart on the prepared baking sheets.
Bake for 8–10 minutes, until cookies are set around the edges but still soft in the center.
While cookies bake, unwrap the mint wafers and cut each in half. When cookies come out of the oven, place one half-mint on top of each cookie and let sit for up to 5 minutes until the mint softens, then gently spread the melted mint over the cookie if desired.