Preparation

Make the yogurt

• Combine the sugar and cardamom in a food processor, and process until powdery, about 10 seconds. Add the pistachios and pulse twice just to very coarsely chop. Transfer  to a large bowl. Whisk in the yogurt, milk, and almond extract. Cover and refrigerate  for at least 4 hours and up to 1 day.
• Stir the mixture, then churn it in an ice cream maker according to the manufacturer’s instructions until it’s the consistency of soft-serve ice cream; if it balls up, let  it sit briefly, and then scrape as much of the mixture off the blades as you can.
• Transfer to a metal loaf pan (preferably 9×5 inches), making sure to scrape any  frozen yogurt off the paddle. Smooth the top, press a piece of plastic wrap directly  on the surface, and cover tightly with foil. Freeze for 4 hours for optimal scooping.  If freezing for much longer, let sit at room temperature until scoopable before  serving.

Make the caramel sauce

In a medium saucepan, combine the sugar, corn syrup, and 1/4 cup water, and cook over  medium-low heat, stirring occasionally, until the sugar dissolves, 2 to 3 minutes.  Increase the heat to medium high and cook without stirring until the sugar begins to  turn golden brown on the edges. Gently swirl the pan over the heat to even out the  color, and cook until the sugar turns golden amber, 8 to 10 minutes. Remove from the  heat. Slowly pour in the cream (be careful—the mixture will bubble vigorously). Stir  with a wooden spoon or heatproof spatula; if there are solid bits, set the pan over  medium-low heat and stir gently until the caramel is smooth. Stir in the butter and  1/2 tsp. salt. Let cool. (The sauce will keep, covered and refrigerated, for 1 month.  Let it come room temperature before using.) Serve drizzled over the frozen yogurt, sprinkled with the sea salt, if you like.