Preheat the oven to 350°F. Lightly grease a 9" x 13" cake pan., Beat the butter and brown sugar together till smooth., Add the eggs, beating till smooth., Stir in the buttermilk and vanilla extract., Add the baking soda, salt, and flour to the wet ingredients, beating until thoroughly combined., Pour the batter into the prepared pan., Bake the cake for 30 to 35 minutes, until the top feels set. The cake will be golden brown and just beginning to pull away from the edges of the pan. Towards the end of the baking time, prepare the topping., Stir the butter and the sugar together. Add the milk, salt, and pecans. The glaze will be thick but pourable., Top the baked cake with the topping and gently spread it in an even layer across the entire surface. Return to the oven for another 10 minutes., Remove the cake from the oven. The topping will look very runny. You can eat the cake hot, with the glaze still gooey; or let the cake sit at room temperature for a few hours, by which time the glaze will have set.