Combine the milk, mayonnaise, cream, sugar and coffee in a medium saucepan. Whisking, bring to a simmer over medium heat and cook until thickened, 4 to 5 minutes. Remove from the heat, add the chocolate chips and vanilla, and whisk until the chocolate is fully melted. Divide evenly between six 115ml mason jars. Chill in the refrigerator until set, about 3 hours. Garnish with berries and serve.