Heat the milk, sugar and salt until it just barely starts to simmer. Remove from heat, stir in Sichuan peppercorns and cover with a lid. Steep for 1 hour. Then strain the milk to remove the peppercorns, and return milk to the pan.
Create an ice bath by nesting one large bowl into a larger bowl filled with ice and water. Add the heavy cream to the top bowl.
In another bowl or large measuring cup, whisk the egg yolks until mixed. Reheat the milk, then slowly pour some of the milk into the egg yolks, whisking well to combine. Continue adding milk and whisking until you've added about half of the milk. Then add the milk and egg mixture to the pan with the remaining milk. Cook over low heat, stirring constantly, until it thickens and the custard coats the back of your spoon or rubber spatula.
Strain the custard through a fine-mesh strainer into the waiting bowl of heavy cream, and mix well. Refrigerate several hours until completely cooled or, preferably, overnight.
Add the cooled custard to an ice cream maker and churn per its instructions, until the ice cream is a soft-serve consistency. If you're making Sichuan pepper ice cream without additions, simply transfer the ice cream to a 1 quart/liter container and freeze. If you are adding the optional brown sugar sesame swirl, quickly layer the ice cream and the swirl into the container. Spoon ⅓ of the ice cream into the bottom of the container in an even layer. Drip and drizzle the sesame swirl over that layer. Repeat 2 more times, ending with a sesame swirl on top. Freeze for several hours.