Adjust rack to middle position and heat oven to 400 degrees.
Grease a 1 1/2-quart porcelain soufflé dish with 1-tablespoon butter, making sure to coat all interior surfaces. Stir together 1/4 cup sugar and cocoa in small bowl; pour into buttered soufflé dish and shake to coat bottom and sides with thick, even coating. Tap out excess and set dish aside.Whisk flour, 1/4 cup sugar, and salt in small, heavy-bottomed saucepan. Gradually whisk in milk, whisking until completely smooth with no lumps. Bring mixture to boil over high heat, whisking constantly until thickened and mixture pulls away from sides of pan, about 3 minutes.Scrape mixture into medium bowl; whisk in remaining 2 tablespoons butter until combined. Whisk in yolks until incorporated; stir in orange zest and Grand Marnier.
This is your soufflé base. Step 1 is done!
In bowl of standing mixer fitted with whisk attachment, beat egg whites, cream of tartar, and 1 teaspoon sugar at medium-low speed until combined, about 10 seconds. Increase speed to medium-high and beat until frothy and no longer translucent, about 2 minutes. With mixer running, sprinkle in half remaining sugar; continue beating until whites form soft billowy peaks, about 30 seconds. With mixer still running, sprinkle in remaining sugar and beat until just combined, about 10 seconds. The whites should form soft peaks when beater is lifted, but should not appear Styrofoam-like or dry.Using rubber spatula, immediately stir one-quarter of beaten whites into soufflé base to lighten until almost no white streaks remain. Scrape remaining whites into base and fold in whites until mixture is just combined.
Gently pour mixture into prepared dish and run index finger around the inside of the dish. This little trick will help the soufflé rise properly.Bake until surface of soufflé is deep brown, center jiggles slightly when shaken, and soufflé has risen 2 to 2 1/2-inches above rim of dish, 20 to 25 minutes.
Serve immediately with fresh whipped cream.
From Cook’s Illustrated