Serves 12 to 16
Line two 8 1/2-by-4 1/2-inch loaf pans with plastic wrap, allowing 4 inches of plastic wrap to hang over sides of pan.
In the bowl of a food processor, combine chocolate and meringue cookies and process until mixture is the texture of coarse breadcrumbs. Set aside.
In the top of a double boiler (off of the heat), whisk egg yolks, 1/2 cup sugar, and the chestnut puree until smooth. (The puree will have flecks of chestnuts in it and therefore will not be silky smooth.) Place the double boiler over simmering water and beat mixture with a handheld mixer on medium speed until pale yellow and fluffy, forming thick ribbons when beaters are lifted, about 4 to 5 minutes. Remove top of the double boiler and transfer to top of an ice bath, whisking until mixture is cold.
In the bowl of a standing mixer with the whisk attached, whip egg whites until foamy and soft peaks begins to form. While continuing to beat, add the remaining 1/2 cup sugar, beating until whites are glossy and stiff peaks form.
In a separate bowl, using a handheld mixer, whip heavy cream with vanilla at medium-high speed until stiff peaks form.
Gently fold about 1/3 of the beaten egg whites into chestnut mixture using a large rubber spatula. Fold remaining egg whites into mixture, taking care not to deflate egg whites. Gently fold whipped cream and brandy into chestnut and egg white mixture in the same fashion.
Spread 2 tablespoons chocolate-meringue mixture over bottom of each plastic-lined pan. Spoon 2 cups chestnut cream over chocolate and meringue, being careful to keep mixtures in two separate layers. Spread another 2 tablespoons chocolate-meringue mixture on chestnut cream. Spoon another 2 cups chestnut cream on top and smooth surface with a rubber spatula. Fold plastic over top of pan and place in freezer.
Freeze for at least 8 hours before serving. Repeat the process with the second loaf pan and the remaining chestnut cream and chocolate-meringue mixture.
To serve, remove semifreddo from freezer and allow it to soften for just a few minutes. Gently unwrap the plastic and carefully lift semifreddo out of pan. Completely remove plastic and invert semifreddo so that chocolate and meringue is on top. Using a slicing knife, cut semifreddo into 3/4- to 1-inch-thick slices. Serve immediately.
Cook's Note: Store-bought meringue cookies may be substituted for homemade meringue cookies. Semifreddo will keep for up to 1 week in the freezer.