Preheat your oven to 350 degrees F (175 degrees C). Line a half sheet pan (18×13 inch) with parchment paper or coat it well with nonstick spray.

In a medium saucepan, melt 1 cup of salted butter and 1 cup of water over medium-high heat. Stir in 2 cups of brown sugar and bring to a rolling boil. Remove from heat and let it cool for a few minutes.

Once cooled slightly, add 2 teaspoons of vanilla extract to the mixture and stir to combine.

In a separate bowl, combine 2 cups of flour with 1 teaspoon of baking soda and 1/2 teaspoon salt.

In a small bowl, whisk together 2 large eggs and 1/2 cup of sour cream, then add this mixture to the batter. Stir until fully combined.

Pour the batter into the prepared pan and bake for about 17-20 minutes, or until a toothpick inserted in the center comes out clean.

For the icing, melt 3/4 cup salted butter, 3 and 1/2 cups brown sugar, 1/2 cup plus 1/3 cup evaporated milk, and 1/4 teaspoon salt over medium heat.

Stir while bringing to a boil until thickened, then remove from heat and mix in 1 and 1/2 teaspoons vanilla extract.

Cool the icing by placing the saucepan in an ice bath while stirring. Once slightly thickened, pour it over the baked cake. Allow the icing to set before slicing and serving.