In a saucepan, bring the maple syrup to a boil with the butter, vanilla, and salt. Cook until a candy thermometer reads 146 °C (294 °F). Watch carefully, because the temperature rises quickly at around 130 °C (260 °F). This step must be done quickly. Remove immediately from the heat and stir in the baking soda. Quickly pour onto the baking sheet. With a spatula, thinly spread the maple caramel to about a 30-cm (12-inch) square. Allow to cool completely.