How to Make It
Preheat oven to 325°F. Grease a 8-inch square baking pan with butter.
Place eggnog in a microwave-safe bowl, and microwave on MEDIUM (50% power), in 15-second intervals, until lukewarm, about 1 minute and 15 seconds. Whisk in bourbon and vanilla; set aside.
Beat egg whites and cream of tartar in the bowl of a heavy-duty stand mixer fitted with whisk attachment on high speed until stiff peaks form, about 4 minutes. Transfer egg white mixture to a separate bowl, and set aside.
Wipe mixer bowl clean; add egg yolks and 1 1/2 cups of the powdered sugar, and beat on high until thick and very pale yellow, about 2 minutes. Add melted butter, and beat until fully combined, about 30 seconds.
Add flour to egg yolk mixture, in 3 additions, beating just until blended after each addition and stopping to scrape down the sides of bowl after the last addition to fully combine. With mixer running on low, gradually add eggnog mixture until combined. (Batter will be very runny.)
Using a rubber spatula, fold one-fourth of egg white mixture into egg yolk mixture; repeat procedure with remaining egg white mixture. (Batter should still be runny, with a few egg white lumps.)
Pour batter into prepared pan, and bake in preheated oven until top is browned and middle is slightly jiggly, 50 to 55 minutes. Cool completely on a wire rack, about 3 hours. Or cool on wire rack 30 minutes, transferring to refrigerator to chill at least 1 hour. Cut into 9 squares, and dust with nutmeg and remaining 1/4 cup powdered sugar.