01 - In a medium saucepan, combine the heavy cream, whole milk, granulated sugar, and salt. Heat over medium heat, stirring continuously until the sugar has fully dissolved and the mixture begins to steam. Do not allow it to reach a boil.
02 - Place the egg yolks in a medium bowl and whisk lightly. Slowly stream approximately ½ cup of the hot cream mixture into the yolks while whisking vigorously to temper them. Gradually pour the warmed yolk mixture back into the saucepan, whisking constantly to combine.
03 - Cook the mixture over low heat, stirring constantly with a spatula or wooden spoon, until it thickens enough to coat the back of a spoon—about 5 to 7 minutes. Maintain gentle heat and never allow the custard to boil.
04 - Remove the saucepan from heat. Stir in the raspberry extract, lemon extract, vanilla extract, and blue food coloring until completely incorporated. Adjust the amount of food coloring to achieve your preferred shade of blue.
05 - Pour the custard through a fine mesh sieve into a clean bowl to remove any curdled bits. Allow it to cool to room temperature, then cover tightly and refrigerate for at least 4 hours or overnight for the best results.
06 - Transfer the chilled base to an ice cream maker and churn according to the manufacturer's instructions until the mixture is thick, creamy, and has the consistency of soft-serve ice cream.
07 - Scoop the churned ice cream into a lidded freezer-safe container. Freeze for a minimum of 2 hours to allow it to firm up before scooping and serving.