Shape the dome: Line a 5- to 6-cup bowl with plastic wrap, leaving overhang. Soften the ice cream 10 minutes until just pliable, pack firmly into the lined bowl pressing out air gaps, smooth the top, fold the overhang over, and freeze at least 2 hours until rock solid.

Boil the syrup: Combine the sugar and 1/3 cup water in a small heavy saucepan; stir once to wet. Bring to a boil over medium-high heat and do NOT stir. Boil 3 to 4 minutes until a candy thermometer reads 240°F (soft-ball stage) — bubbles turn thick and slow, syrup sheets off a spoon.

Whip the meringue: While the syrup boils, whip the egg whites with the lemon juice to soft peaks. With the mixer running, stream the 240°F syrup in a thin thread into the whites. Whip on high 4 to 5 minutes until glossy, stiff, and cool; it passes the bowl-upside-down test.

Cut the sponge: Slice the pound cake into 1/2-inch slabs and cut two 6-inch rounds. Brush one round with the 2 tablespoons of cognac, if using.

Assemble: Set the soaked round on a serving platter. Unmold the frozen dome onto it (warm the bowl with your palms, invert, peel off plastic) and press the second round gently on top. If anything softens, freeze 10 minutes before continuing.

Coat with meringue: Working quickly, spread the meringue over the entire dessert at least 1/2 inch thick, right down to the platter, with no gaps. Pull tall swirls and peaks; run a fork through for ridges. Freeze 30 minutes to set.

Flambe and serve: Warm the 1/4 cup cognac until it just steams (about 120°F, never boil). At the table, pour into a warm metal ladle, ignite with a long match held away from the body, and pour the flaming liquor over the meringue. Flames die in 15 to 30 seconds; slice with a hot, dry knife and serve immediately.