• Preheat the oven to 350 degrees F (175 degrees C). Grease a 10x15 inch sheet pan and line with parchment paper if desired (for easy cake removal).
• In a saucepan over medium heat, melt the unsalted butter. Then add the brown sugar and granulated sugar. Bring the mixture to a gentle boil. Stirring constantly until the sugar dissolves.
• Remove the mixture from the heat, stir in the vanilla extract. Allow it to cool while you prepare the remaining ingredients.
• In a large mixing bowl add the all purpose flour, baking powder, baking soda, and salt. Using a whisk, stir to combine. Set aside.
• In a medium bowl, add the sour cream and eggs. Stir to combine.
• Add the cool water to the butter sugar mixture in the saucepan to help cool the mixture down, stir to combine.
• Add the butter, sugar, and water mixture to the bowl with the sour cream and eggs. Stir to combine.
• Add the wet ingredients to the dry ingredients. Stir until just combined.
• Pour the batter into the prepared pan and using a spatula, even out the mixture.
• Transfer to the preheated oven and bake at 350 degrees F for 20-25 minutes, until a toothpick inserted comes out clean.
• About 10 minutes into the baking process, prepare the frosting.

Make the frosting

• In a saucepan over medium heat, melt the butter.
• Add the brown sugar. Bring to a gentle boil and cook for 2 minutes, stirring constantly so the mixture doesn’t burn or boil over.
• Remove from the heat and stir in the cream, salt, and vanilla extract. Stir to combine.
• Whisk in the powdered sugar until smooth and pourable.
• Immediately pour frosting over the warm cake and spread quickly.
• Let it sit for 20-30 minutes before slicing.