1. In a saucepan, heat the milk and butter until the butter melts.

2. In a separate bowl, whisk together eggs and sugar until well combined.

3. Slowly pour the heated milk mixture into the egg mixture while whisking continuously.

4. Return the mixture to the saucepan and cook over low heat, stirring constantly, until it thickens into a custard-like consistency.

5. Remove from heat and stir in the port wine.

6. Let the mixture cool completely before churning it in an ice cream maker according to the manufacturer's instructions.

7. Once the ice cream is ready, scoop it into bowls and serve with a drizzle of extra port wine on top.