Fill a shaker with ice and add the whiskey, orgeat, and lemon juice. Shake vigorously for 5 seconds. Strain over pebble ice (pro tip: You can buy 1 lb/450 g bags of pebble ice at a Sonic Drive-In). Dash the bitters around the top of the ice and garnish with a mint sprig and freshly grated nutmeg.
Chestnut OrgeatTo roast chestnuts, cut an “X” through the shell of the rounded side, making sure to completely pierce the outer shell and inner skin. Roast over a fire (or for 20 to 30 minutes in an oven at 400 degrees Fahrenheit until the shells begin to caramelize and split at the incision point. Use your fingers to peel away the outer shell and papery skin. Sometimes the skin doesn’t come off easily, in which case you may have to chisel away at it with a paring knife. If a little is left behind, don’t sweat it. After peeling, place the chestnuts on a roasting pan and toast in a 350 degree F oven for 10 minutes more.
Next, use a knife or a food processor to coarsely chop the chestnuts. Top with the boiling water, cover, and let sit at room temperature for 3 hours. Strain the liquid through a fine-mesh strainer lined with a tea towel or cheesecloth, being sure to squeeze as much liquid from the nuts as possible (should yield about 2 1/2 cup) and discard the solids. This strained liquid is now chestnut milk.
Add the chestnut milk to a small saucepan along with the sugar and heat, stirring frequently, until the sugar completely dissolves. Move to the refrigerator to cool. Once chilled, add orange blossom water and rye and store in a swing-cap bottle in the refrigerator for up to 1 month.