In a medium saucepan, combine the whole milk, double cream, half the caster sugar, milk powder, honey, Maldon sea salt, and vanilla paste. Heat gently over medium heat until the mixture starts steaming but does not boil.

In a separate bowl, whisk the egg yolks with the remaining caster sugar until fluffy and pale. Slowly pour the hot milk mixture into the egg yolks while whisking continuously.

Return the tempered egg mixture to the saucepan over low heat. Stir steadily for about 5-8 minutes until the custard thickens.

Remove the saucepan from heat and strain the custard through a fine sieve into a clean bowl. Allow to cool at room temperature for about 30 minutes.

While the custard cools, preheat the oven to 180°C (350°F). Toss the chopped rhubarb with sugar, lemon juice, and grenadine. Roast for 15-20 minutes until tender.

Blend the roasted rhubarb mixture until smooth, creating a vibrant puree.

In a large bowl, fold half of the chilled custard into the rhubarb puree. Gradually mix in the remaining custard, ensuring uniform color and flavor.

Transfer the mixture to an ice cream maker and churn until it reaches a soft-serve consistency, typically around 20-25 minutes.

Scoop the churned ice cream into an airtight container and freeze for at least 4 hours or until firm.