Butter and flour a half-sheet pan (18x12) or jelly roll pan (15x10).
Sift flour and salt into large bowl.
Add sugar and whisk until blended.
Combine butter and water in a saucepan.
Bring to a boil and pour over the flour mixture. Whisk until blended.
Add the soda, buttermilk, eggs, and vanilla, and whisk until blended and smooth.
Pour into greased and floured baking pan.
Bake for 12-20 minutes, checking cake after 12.
Cake is done when tester comes out clean but be careful not to overcook.
In large saucepan, place 2½ cups sugar, butter, cream, and a dash of salt. Cook on medium until almost to a rolling boil then pour in sugar syrup that you are making in another skillet.
At the same time, in a small skillet, put ½ cup sugar, stir in 2 tablespoons water, and melt on medium until it becomes an amber color. You don't stir this sugar after the first time, but you will need to shake the skillet every few minutes to help distribute the sugar. This is the sugar syrup that you will pour into the saucepan.
Once you pour in the syrup, cook on medium to medium-high, stirring constantly until soft ball stage on thermometer, about 10 minutes.
Remove pot from heat, stir in vanilla, and let cool for about 15 minutes.
Either transfer to large stand mixer on medium or use hand mixer and whip until it becomes frosting consistency, 15-20 minutes.
If frosting sets too quickly or gets too thick, you can add a little additional cream.
The Chemistry of Fats and Sugars
Replacing unsalted butter with salted butter alters the final structure because commercial dairy manufacturers vary salt ratios wildly between brands. Pastry bakers historically favored sheet formats because baking a thin layer of batter in an 18x12 pan maximizes surface area for rapid evaporation, yielding a tender crumb that differs fundamentally from deep, dense layer cakes. Sifting the all-purpose flour and salt together before adding the sugar prevents pockets of uneven density, while bringing the butter and water mixture to a rolling boil gelatinizes starches slightly before the eggs enter the bowl.
Managing the Dual-Pot Frosting Method
Executing the caramel component requires two separate pans operating simultaneously, demanding absolute focus since molten sugar reaches amber hues rapidly without stirring. Swirl the small skillet containing the half cup of sugar and two tablespoons of water rather than using a utensil, because agitation introduces sugar crystals that ruin the smooth thermal transition. Once the amber syrup merges into the heavy cream and butter base in the large saucepan, cook the mixture until a candy thermometer reads the soft ball stage, which typically takes roughly ten minutes on medium-high heat.
Whipping and Post-Baking Evolution
Allowing the cooked caramel base to cool for precisely fifteen minutes prevents the heat from shocking the butter when the mechanical whisk starts aerating the mass. Whip the mixture on medium speed for fifteen to twenty minutes until it transforms into a spreadable frosting, keeping a splash of heavy cream nearby if the emulsion thickens too quickly. Storing leftovers covered at room temperature for up to two days actually improves the texture, as the moisture redistributes evenly throughout the crumb and softens the crystallized caramel surface.