Maple-Brown-Butter Semifreddo

In a medium bowl, using an electric mixer, beat cream on high until stiff peaks form, 2 minutes; refrigerate. In a small saucepan, heat butter over medium, swirling pan occasionally, until golden brown and most of the foam has subsided, 8 to 10 minutes. Immediately pour into a small bowl. In clean saucepan, bring maple syrup to a boil over medium-high and cook 2 minutes.

In a medium bowl, using an electric mixer, beat egg yolks on high until pale yellow, 1 to 2 minutes. With mixer running, add hot maple syrup in a slow, steady stream, then add brown butter. Beat on high until mixture has cooled to room temperature, about 5 minutes. With a rubber spatula, gently fold in whipped cream. Transfer mixture to a 5-by-10-inch loaf pan and cover with plastic wrap. Freeze until firm, 5 hours (or up to 1 week). Serve topped with hazelnuts, if desired.