Make the buttermilk ice cream base: In a large jug, combine the buttermilk and heavy cream. Set a strainer over a heatsafe container.

In a medium saucepan, whisk together the sugar, milk powder, salt, and xanthan gum. Whisk in the egg yolks and corn syrup until well combined, followed by half (~360g) of the buttermilk mixture. Cook over medium-low heat, stirring and scraping the pot constantly with a heatproof spatula, until the custard is thick enough to coat the back of a spoon and registers 170-180F on a digital thermometer. Immediately strain into the prepared container. Stir in the remaining buttermilk mixture and vanilla extract. Cover and refrigerate until cold (4 -24 hours) before churning.

Make the rhubarb jam: Combine the rhubarb, sugar, salt, vanilla, and lemon juice in a medium saucepan. Cook, stirring frequently, over medium-high heat until boiling. Reduce the heat to medium and continue cooking, stirring frequently, until the mixture is thick and sticky, about 20 to 30 minutes. Transfer to a clean jar and cool to room temperature. Refrigerate until ready to use.

Make the brown sugar oats: Preheat the oven to 325F with a rack in the middle. Line a sheet pan with parchment or a silicone baking mat. In a small saucepan over low heat, melt together butter, sugar, and salt, stirring occasionally. Bring to a boil and cook for 1 minute, then remove from heat and stir in oats. Scrape into a single layer onto prepared pan. Bake for about 20 minutes, stirring every 5 minutes or so, or until oats are golden. Cool completely on a wire rack, then store in an airtight container.

Churn and assemble the ice cream: Before churning, place the container for your ice cream in the freezer. Churn buttermilk base according to the instructions for your machine, until the mixture has the texture of soft serve (about 25 minutes for my machine). Spread a layer of ice cream in the bottom of the pan, then spoon over dollops of rhubarb jam and a generous sprinkle of brown sugar oats. Continue layering up the ice cream, jam, and oats in the pan until you’ve used up all the ice cream. Cover with parchment paper, pressing it to the surface of the ice cream so it adheres, then cover with a lid. Freeze until firm, at least 6 hours. Ice cream will keep for up to 3 months.