Method

• 1.
  Line a medium-size 25cm x 13cm (1.5L) loaf tin with baking paper and pop it in the freezer to chill. Extend the baking paper beyond the edges of the tin, to help ease the semifreddo out once frozen.
• 2.
  Put the cream and chopped chocolate into a large, heavy-based saucepan over very low heat. Warm the mixture, stirring it very regularly (ideally with a flat sauce whisk to get right into the corners of the pan), until the chocolate has melted and the mixture is thick and smooth. Don’t let it get too hot (not remotely near the boil) or the texture will change. Remove the pan from the heat before the last few pieces of chocolate have melted as the residual heat tends to do this perfectly.
• 3.
  Pour the chocolate mixture into a large bowl and leave to cool. Let it chill in the fridge, stirring regularly so it doesn’t set. Meanwhile, cut or break the halvah into small pieces. It will crumble and you’ll end up with a range of sizes. Spread them out thinly on a flat plate or lined baking tray and pop in the freezer.
• 4.
  Once the chocolate mixture is chilled, add the vanilla and use a hand-held electric beater on medium speed to beat until it becomes thick and creamy looking, but not stiff, or it will develop a slightly grainy texture. I often stop a bit before it’s ready and finish off by hand with a balloon whisk, for more control over the final texture (or, if you’d rather, use a balloon whisk all the way through). Gently fold the halvah pieces and 1 tbs of the slivered pistachios into the chocolate mixture, then spoon it into the prepared tin. Alternatively, you can scrape it into a container with a tight-fitting lid and serve in scoops once it’s frozen. Scatter the remaining pistachio nuts over the top and pop it in the freezer overnight.
• 5.
  To serve, ease the semifreddo out of the tin using the baking paper, then cut it into thick slices. Serve on chilled plates. See notes for storing.