How hot should my water be? The hot water in this cake batter should be not quite boiling (175° F or so - about the temperature you’d use for making tea). You can bring water to a boil in a pan or kettle on the stovetop while you prepare the rest of your ingredients, or use an electric tea kettle to heat the water. If you don’t have the time or equipment to heat water this way, you can use water straight from the tap on its hottest setting - it won’t draw out quite as much chocolate flavor, but it will still work!

Please use a kitchen scale! Measuring ingredients by volume (with cups) is notoriously unreliable: For best results, use a kitchen scale and measure your ingredients by weight using the gram measurements above. If you don’t have a scale, measure your ingredients (particularly flour and cocoa) with the scoop-and-level method for best results. 

Equipment notes. Use a rimmed half sheet pan for this cake (it should measure 13” x 18” and be at least 1” tall). If you use a nonstick pan, choose a lighter color if possible (darker pans conduct more heat and can result in crispy edges; if you use a dark pan, reduce the oven temperature to 325° and add an extra 3-5 minutes to the bake time). Choose a sturdy pan that won’t warp in the oven. We used a Great Jones nonstick half sheet pan in these photos. Use parchment paper (not wax paper) to line your sheet pan and help the cake release easily.

Can I make this cake with a stand mixer? Yes! Mix this cake in the bowl of a stand mixer fitted with the paddle attachment. Pause the mixer and scrape down the sides of the bowl once or twice while you mix the hot water into the batter to ensure you work out any lumps.

Storing and freezing this cake. Extra cake slices will last up to 5 days in an airtight container on the counter, although the texture is best if this cake is eaten within 1-2 days of baking. Freeze leftover cake slices in an airtight container in the freezer for up to 3 months; defrost frozen cake on the counter when you’re ready to eat it.

Additions and Substitutions. Use coconut oil in place of some or all of the butter in this recipe (see more tips in our Coconut Oil Cupcakes recipe). You can also use vegetable or canola oil in place of the melted butter in the cake batter. Use hot coffee instead of hot water in the cake batter if you like. Use cooking spray instead of softened butter to grease your sheet pan if you prefer. Use your favorite buttercream frosting in place of the chocolate fudge frosting in this recipe (note: if you use buttercream, let the cake cool completely before frosting). Use chopped walnuts, pecans, or pistachios instead of sprinkles to decorate the cake.

The chocolate fudge frosting in this recipe will start to seize up as the butter in it cools, so work quickly to frost your cake. The frosting will be quite thick and will resemble warm fudge - it won’t look or feel like the traditional buttercream or whipped frostings we’re often used to. If the frosting is too thin (this often happens when too much hot water is added) add extra powdered sugar or let it cool on the counter for a few minutes to thicken it up. If frosting is too thick, stir in an extra splash of hot water or zap it in the microwave in 5-second intervals to loosen it. I love that this frosting develops a rich, fudge-like texture as it cools (it’s one of my most-requested frostings from friends and family!) but you could also use a more traditional buttercream here if you prefer.

This recipe makes a thin layer of frosting. If you'd like a thicker frosting layer, we recommend doubling the frosting recipe.