Melt sugar over medium heat until deep amber (do not stir, swirl pan).
Carefully add boiling water, stir smooth, simmer 3–5 min. Cool. Reserve ½ cup for cake + ½ cup for frosting.
Heat oven to 350°F / 175°C. Grease three 8-inch pans.
Cream butter + sugar 3–5 min. Add eggs one at a time.
Whisk dry ingredients. Alternate dry ingredients + milk into batter.
Mix in burnt sugar syrup + vanilla. Bake 25–30 min. Cool completely.
Boil butter, brown sugar, syrup & milk 2 minutes. Cool until warm, then beat in powdered sugar, salt & vanilla.
Frost cake quickly before it sets.
Melt ¾ cup butter. Boil brown sugar + cream + salt.
Beat into melted butter with powdered sugar. Add remaining butter 1 tbsp at a time. Rest 10–30 min until spreadable.
Assemble Fill and frost cake with traditional burnt sugar frosting. Optional: cover outside with penuche frosting. Finish with rustic swoops and swirls.
