Combine butter, sugar, corn syrup and salt in a large pot over medium heat. Stir occasionally until ingredients are melted together.
Once the mixture has melted, increase heat to medium-high and bring the mixture to a boil. Once the mixture boils, slowly add the evaporated milk a little bit at a time, stirring after each addition and letting it come back to a boil after each addition. It should take about 10-15 minutes to add the entire can.
Using a candy thermometer, continue to boil the caramel until it registers 238° on the thermometer. Remove the pot immediately from the heat and let sit off the heat for 5 minutes.
Pour caramel over pecans and marshmallow and let sit until cooled completely (approximately 3 hours).
Once the caramels and marshmallow are completely cooled and set up, turn out onto a cutting board.
Trim the edges to make a clean cut on all sides and then cut into approximately 1 ½" pieces.