Stir sugar, glucose and 125ml water in a saucepan over medium-high heat to dissolve sugar, then cook until mixture reaches 121C on a sugar thermometer (5-10 minutes).
Meanwhile, when sugar thermometer reaches 105C, start whisking yolks in an electric mixer on high speed until pale, then reduce speed to low. Slowly add syrup and whisk until cooled to room temperature (8-10 minutes). Transfer half the mixture to a bowl, whisk in mascarpone, then whisk in remaining egg mixture. Fold in cream and buttermilk, then thoroughly fold in walnut oil. Transfer to a 20cm x 30cm metal pan brushed very lightly with oil and lined with plastic wrap and freeze until frozen (3-4 hours). Just before serving, turn out and cut into 12 pieces.
Meanwhile, for barley crisps, preheat oven to 140C. Cook barley in a large saucepan of boiling water over medium-high heat until tender (35-40 minutes; add more water if necessary). Drain (reserve 175ml cooking liquid), transfer barley, reserved cooking liquid, remaining ingredients and 1 tsp sea salt flakes to a blender and blend until smooth. Spread thinly to three 20cm x 30cm rectangles on oven trays lined with baking paper, then bake until dry (1¾-2 hours; turn crisps over after 1 hour). Break into rough pieces, sandwich pieces of parfait between crisps and serve.
This recipe is from the June 2012 issue of Australian Gourmet Traveller.