Freeze your ice cream machine bowl in advance
Heat Milk and Cream until just simmering
Remove from heat, cover, and let steep
In a stand mixer, whisk yolks, sugar, cornstarch, and a pinch of salt until pale yellow and thick (4–5 minutes)
Slowly temper hot milk into yolk mixture, whisking constantly
Return mixture to the pan
Stir constantly over medium heat until thickened and just beginning to bubble
Chill custard quickly in an ice bath or refrigerator until fully cooled
Whisk ½ to 1 cup of cooled peach-bourbon reduction into the chilled custard
Pour the flavored custard into your ice cream machine and churn according to instructions until soft-serve consistency
Gently fold in the frozen peach cubes
Transfer ⅓ of the gelato to a chilled container
Drizzle some of the reserved reduction over it
Repeat layering until all gelato is in the container
Run a spatula through once or twice in a zigzag to create ribbons (don’t overmix)
Cover and freeze 2–4 hours until just firm