Make the ice cream center: Line a 2 1/2-quart bowl that is 9-inches across in diameter with plastic wrap so that the wrap comes over the sides of the bowl. Soften the ice cream (either in a microwave or by letting it sit at room temperature until soft). Using a rubber spatula, spread the ice cream into the bottom of the bowl evenly. It should fill the bowl about 3/4 full. Smooth the top. Sprinkle ice cream with nuts or chocolate chips if using. You can make a multi-flavored Alaska by using different flavors of ice cream, freezing each layer before adding a new one. Cover the ice cream and bowl with plastic wrap and place into the freezer until hardened, at least 4 hours, or overnight.
Make the brownie base: Preheat oven to 375 degrees F. Lightly grease the bottom and sides of an 8-inch round cake pan. Line the pan with aluminum foil so that the excess foil comes up and over the sides of the pan. Lightly grease the inside of the foil in the pan.
Melt chocolate and butter in a metal bowl set over a saucepan of barely simmering water, stirring, until smooth. Remove bowl from pan and whisk 3/4 cup sugar into chocolate mixture (it will seem a little bit grainy). Whisk in eggs and salt, then sift cocoa over, and whisk until just combined.
Pour batter into pan, spreading evenly, and bake in middle of oven until a tester comes out with a few crumbs adhering, 25 to 35 minutes. Cool brownie in pan on a rack 5 minutes, then carefully lift foil and cake out of the pan together by lifting the excess foil on the sides of the pan. Cool to room temperature.
To assemble: Remove the bowl with the ice cream from the freezer and remove the plastic wrap covering the top. Open the plastic wrap covering the ice cream so that the ice cream is exposed. Carefully invert the brownie on the foil and place it directly on top of the ice cream. Press down slightly so it adheres. Peel off the foil and wrap the plastic wrap over the brownie. Tightly cover the bowl with another piece of plastic wrap and return to the freezer for about 30 minutes.
Preheat oven to 450 degrees F and set the rack in the middle of the oven.
After ice cream and cake have been freezing 20 minutes, beat egg whites and a pinch of salt with an electric mixer until foamy, then add lemon juice, and continue to beat until whites hold soft peaks. Gradually add the sugar, beating, and continue beating until whites just hold stiff, glossy peaks.
Fill a large bowl halfway with hot water. Remove ice-cream base from freezer and briefly dip it into the bowl with hot water to loosen it, being careful not to get any water into the bowl. Invert the ice-cream base/bowl onto an oven-safe, decorative platter (or sheet tray). Lift the bowl off of the ice-cream mound and remove the plastic wrap. With an offset spatula, mound meringue over it, spreading to cover brownie layer completely. Form decorative peaks in the meringue by making short vertical strokes with the spatula. Bake in middle of oven until golden brown, about 6 minutes. Serve immediately.
Cooks' notes: The base can be assembled 1 week ahead and frozen, tightly wrapped. Make meringue and, after topping base, bake Alaska 10 minutes.
*RAW EGG WARNING
Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the slight risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell.
Adapted from Gourmet Magazine