Lemon-Blackberry Semifreddo

Make lemon curd: Put egg yolks, eggs, 3/4 cup sugar, and lemon juice in a medium heatproof bowl; set over a pan of simmering water. Whisk until mixture is very thick and smooth and temperature registers 165 degrees on an instant-read thermometer, about 5 minutes. Remove from heat; whisk in butter. Pour through a fine sieve into a medium bowl; cover with plastic wrap, pressing wrap directly on surface of curd. Refrigerate until cool and firm, about 1 hour.

Put egg whites in the bowl of an electric mixer fitted with the whisk attachment; beat on low speed. Meanwhile, bring 1/3 cup sugar and 2 tablespoons water to a boil in a small saucepan over high heat. Swirl pan to dissolve sugar. Wash down sides of pan with a wet pastry brush to prevent crystals from forming. Boil until syrup registers 248 degrees on an instant-read thermometer. Raise mixer speed to medium high; beat until egg whites have formed almost-stiff peaks. With mixer running, slowly pour in syrup; beat until mixture has cooled and stiff, glossy peaks have formed, about 4 minutes. Set meringue aside.

Stir mascarpone in a large nonreactive bowl with a rubber spatula until smooth. Fold in lemon curd and lemon zest. Fold in meringue.

Pulse berries and remaining 2 tablespoons sugar in a food processor until berries are broken up but not smooth. Fold into lemon mixture, leaving it slightly swirled. Freeze until slightly firm, about 20 minutes. Use immediately.