Line a lightly oiled 6-cup loaf pan with plastic wrap and set aside.
In the top of a double boiler or in a metal bowl set over a pan of simmering water, combine the chocolate and butter and melt until smooth, stirring occasionally. Remove from the heat and fold in the pistachios. Let cool.
In a bowl fitted with an electric mixer, beat the heavy cream until stiff peaks form. Whisk about 1/3 of the cream into the cooled chocolate, then gently fold in the remaining whipped cream. Pour the chocolate mixture into the prepared pan, smoothing the top with a rubber spatula. Cover with plastic wrap and chill it overnight.
Unwrap the terrine, invert onto a serving platter, and shake gently to release. Remove the plastic wrap from the top and let sit at room temperature about 15 minutes to soften. Slice into 1/2-inch to 3/4-inch slices and serve with the Orange Creme Anglaise on the side, and garnish with the candied almond slivers.
Yield: 10 to 12 servings