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1
Preheat oven to 375F.
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2
Butter an 8-inch square baking pan, then line bottom with wax paper and butter paper.
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3
Melt chocolate and butter in a metal bowl set over a saucepan of barely simmering water, stirring, until smooth.
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4
Remove bowl from pan and whisk 3/4 cup sugar into chocolate mixture.
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5
Whisk in whole eggs and salt, then sift cocoa over and whisk until just combined.
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6
Pour batter into baking pan, spreading evenly, and bake in middle of oven until a tester comes out with a few crumbs adhering, 20 to 25 minutes.
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7
Cool cake in pan on a rack 5 minutes, then invert onto rack and cool completely.
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8
Cut cake into 6 equal pieces, then arrange 1 piece in each gratin dish, trimming to fit.
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9
Top each piece of cake with a large scoop of ice cream (about 1/2 cup), then freeze, covered, just until ice cream is hard, about 25 minutes (do not let ice cream become rock hard unless making ahead see cooks' note, below).
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10
Preheat oven to 450F.
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11
Beat egg whites and a pinch of salt with an electric mixer until they just hold soft peaks.
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12
Add remaining 1 1/2 cups sugar a little at a time, beating at high speed, and continue beating until whites just hold stiff, glossy peaks, about 5 minutes in a standing mixer or about 12 minutes with a handheld.
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13
Remove gratin dishes from freezer and mound meringue over ice cream and cake, spreading to edge of gratin dish.
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14
Bake on a baking sheet in middle of oven until golden brown, about 6 minutes.
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15
Serve immediately.
