Whisk together first 3 ingredients in a large heavy saucepan. Gradually whisk in milk and half-and-half. Cook over medium heat, stirring constantly, 8 to 10 minutes or until mixture thickens slightly. Remove from heat.
Whisk egg yolk until slightly thickened. Gradually whisk about 1 cup hot cream mixture into yolk. Add yolk mixture to remaining cream mixture, whisking constantly.
Pour mixture through a fine wire-mesh strainer into a bowl, discarding solids. Cool 1 hour, stirring occasionally. Place plastic wrap directly on cream mixture; chill 8 to 24 hours.
Pour mixture into freezer container of a 1 1/2-qt. electric ice-cream maker, and freeze according to manufacturer’s instructions; stir in Key lime zest, Key lime juice, and crushed graham crackers halfway through freezing. Let stand at room temperature 5 to 10 minutes before serving.
*Granulated sugar may be substituted.
Note: We tested with Whey Low 100% All Natural Granular Sweetener for Ice Cream. Get 20% off any time you order from wheylow.com. Coupon code: SouthrnLvg
Grease and line 2 baking sheets with baking parchment.
In a food processor, whizz the flour, baking powder and butter until incorporated. Add 30g of the cheese and rosemary setting the remaining 30g of cheese aside.
Add 4 tbsp. water and whizz again to make a dough. (You might need a bit more, so add more liquid carefully as you want it soft not sticky) Turn out onto a floured surface and bring the dough together with your hands.
To roll out the dough, divide it into two and roll out half at a time. It needs to be really thin, so roll it out between two pieces of baking parchment, that way it won’t stick, will be easier to turn and you’ll need less flour.
Cut into about 20 squares and lift them onto your prepared trays. Brush with water and sprinkle with the remaining parmesan. You can also sprinkle sparingly with a little sea salt. Repeat these stages until all you have used up all the dough.
Bake in the preheated oven for 13 minutes. You want the biscuit to be crisp but not too coloured.