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1
Preheat oven to 200F.
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2
Beat egg white with a pinch of salt in a small deep bowl with an electric mixer at medium speed until it holds soft peaks.
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3
Beat in sugar 1 tablespoon at a time and continue to beat until meringue holds stiff glossy peaks, 3 to 5 minutes.
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4
Fold walnuts into meringue, then spread into a 9- by 6-inch rectangle (1/4 inch thick) on a parchment-lined baking sheet.
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5
Bake meringue in middle of oven until crisp, about 1 hour.
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6
Slide meringue (on parchment) onto a rack and cool completely.
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7
Peel off paper and break meringue into 1/2-inch pieces.
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8
Line bottom of molds with rounds of wax paper or parchment.
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9
Cut 10 (10-inch-long) strips of wax paper or parchment (wide enough to cover sides of molds) and arrange, ends overlapping, around insides of molds.
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10
Put lined molds in a baking pan (for transferring to freezer).
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11
Puree blackberries in a food processor, then force through a fine-mesh sieve into a bowl, discarding seeds.
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12
Divide puree evenly between 2 bowls.
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13
Stir 2 tablespoons sugar and 1 teaspoon lemon juice into 1 bowl (for sauce) and chill, covered.
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14
Stir remaining 2 teaspoons lemon juice into other bowl (for semifreddo).
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15
Beat eggs with a pinch of salt and remaining 1/3 cup sugar in a metal bowl with a handheld mixer at medium-high speed until doubled in volume, about 5 minutes.
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16
Set bowl over a saucepan with 1 inch of simmering water and beat until mixture registers 160F on thermometer, 6 to 8 minutes.
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17
Remove bowl from heat and chill mixture until cool, about 10 minutes.
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18
Fold in blackberry puree (reserved for semifreddo) gently but thoroughly.
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19
Beat cream with cleaned beaters until it just holds stiff peaks and fold gently into blackberry mixture, then fold in walnut meringue pieces.
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20
Spoon into molds, smoothing tops if necessary, and cover with plastic wrap.
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21
Freeze semifreddi until firm, at least 2 hours.
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22
To serve, dip each mold in a bowl of hot water 3 seconds, then wipe dry and invert onto chilled dessert plates, using paper to help pull each semifreddo from mold.
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23
Remove paper and let desserts stand at room temperature 10 minutes to soften.
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24
Serve with blackberry sauce.
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25
*Available at some cookware shops and Bridge Kitchenware (800-274-3435 or 212-838-1901).
