Preheat the oven to 350 degrees. Spray a 13- by 9-inch baking dish with cooking spray and dust lightly with flour. Set aside.

Combine the buttermilk and butter in a small saucepan set over medium heat. Heat, stirring often, until the butter is melted. DO NOT BOIL! If the mixture starts to boil, turn the heat down.

Pour that mixture into a large mixing bowl and let cool 5 minutes. Add the brown sugar, eggs, and vanilla extract. Beat with an electric mixer set to medium speed for several minutes, until well combined.

Whisk the flour, baking soda, and salt in a small bowl and, with mixer set to low, gradually beat into the butter-egg mixture until just combined. Stir with a spatula a few times to make sure all of the dry ingredients are incorporated.

Scrape into the prepared baking dish and smooth the top. Bake 22 to 32 minutes, until a cake tester inserted into the center of the pan comes out clean.

Remove from the oven and set on a wire rack, then immediately start the frosting and heat your oven’s broiler.

Combine all the frosting ingredients in a small saucepan and set the pan over medium heat. Bring to a slow boil, stirring often.

Once the frosting boils, cook for 3 to 5 minutes, until the mixture just starts to thicken.

Pour over the cake, then put the cake in the oven 4 inches below the broiler. Broil 30 seconds to 1 minute, until the frosting starts to bubble and turns light golden brown. Don’t walk away from the oven because this will happen quickly.

Uneaten cake can be stored at room temperature, covered, for about 2 days, in the fridge, covered, for about 3 days, or in the freezer, tightly wrapped, for up to 3 months.