In a bowl with an electric mixer, blend walnuts, 3/4 of the butter, sugar, flour, zest and egg until combined well, and press into an 11 inch tart pan with removable bottom. Freeze shell 15 minutes. While shell is freezing, preheat oven to 375ºF. Bake shell in middle of the oven 25-30 minutes, or until golden brown. Cool on a rack. Remove side of pan and transfer shell to a plate. Fill shell with 2 1/2 cups of raspberries. In a large metal bowl set over a saucepan of barely simmering water, melt the chocolate. Remove bowl from the heat and whisk in the cream and the remaining butter, whisking until smooth. Spread chocolate mixture over the raspberries, smoothing top and chill, covered, for 3 hours or overnight. Garnish tart with whipped cream and remaining 1/2 cup of raspberries. Serve at room temperature.