Coffee Semifreddo

Combine coffee and sugar in a saucepan over medium heat and bring to a boil, stirring occasionally so sugar does not stick to the bottom of the pan. Remove from heat.

Whisk egg yolks in a heatproof bowl until frothy. Add sweetened coffee 1 tablespoon at a time, whisking vigorously to prevent egg yolks from curdling.

Place bowl on top of a pot with simmering water, making sure the bottom of the bowl does not touch the water. Continue whisking vigorously until mixture thickens and gets creamy, 4 to 6 minutes.

Remove bowl from heat and place over a bowl of ice; whisk until cooled. Fold in mascarpone cheese.

Beat cream in a chilled glass or metal bowl with an electric mixer until stiff peaks form. Stir 2 tablespoons of whipped cream into the coffee mixture to thin it out. Fold in remaining cream carefully with a spatula until just combined. Fold in walnuts.

Pour mixture into 4 serving or ice cream glasses and freeze until firm, at least 5 hours.