Combine the sugar and cornstarch in a medium saucepan over low heat. Slowly add the milk, whisking to blend. Bring to a bare simmer, then stir in the butter, chocolate, vanilla, and a pinch of table salt. Remove from the heat and continue stirring until the chocolate has melted uniformly. Pour into 4 small glasses or ramekins and place in the fridge for at least 2 hours. Before serving, drizzle the puddings with a bit of olive oil and top each with a pinch of sea salt.