Heat the oven to 325°F. Set eight 8-oz. ramekins or custard cups in a large 2- to 3-inch-deep pan. (If you don’t have one large pan, use two smaller ones; you want to leave a little space between the ramekins for even cooking.)
In a medium saucepan, melt the butter and brown sugar together over mediumlow heat, stirring occasionally, until the sugar is melted and emulsified (it will still be slightly grainy and look like wet sand), about 15 minutes. Remove the pan from the heat and stir in the Scotch—stand back and be careful; adding the whisky will make the mixture spatter. In a small saucepan, heat the cream and milk just until they start to boil; stir into the brown sugar mixture. In a medium bowl, lightly whisk the egg yolks. Add a ladleful or so of the cream mixture to the yolks, whisking to combine. Continue adding the cream a ladleful or so at a time, whisking. Pass the mixture through a fine sieve and then divide it evenly among the ramekins, leaving at least 1/2-inch space at the top. Position the pan on the oven rack and pour enough hot water into the pan to come halfway up the sides of-the ramekins. Cover the pan with foil, poking a couple of holes in the corners. Bake for 30 to 35 minutes, checking after 30 minutes, until there’s a slight jiggle in the center of each custard cup (not a wavelike motion) when nudged (ramekins baked in a glass baking dish will take more time than those in a metal pan). If the custards need more cooking, cover the pan again with foil, checking for doneness every 3 minutes or so. Remove the pan from the oven, take the ramekins out of the water bath right away, and let cool at room temperature. When cool, transfer to the refrigerator to chill for at least 2 hours or overnight.
Whip the 3/4 cup cream with the granulated sugar to medium peaks. Serve each custard with a dollop of the whipped cream and a sprig of mint.