Step 1

FOR THE CAKE: Adjust oven rack to middle position and heat oven to 350 degrees. Grease 2 (8-inch) square cake pans with baking spray with flour. Whisk flour, baking soda, baking powder, and salt together in bowl; set aside. In second bowl, whisk hot water, cocoa, and vanilla until smooth; set aside.

Step 2

Using stand mixer fitted with paddle, beat butter and sugar on medium-high speed until light and fluffy, about 3 minutes. Add eggs, one at a time, and mix until incorporated. Reduce speed to low and add flour mixture in 3 additions, alternating with buttermilk in 2 additions, until combined. Slowly add cocoa mixture until incorporated.

Step 3

Divide batter evenly between prepared pans. Bake until toothpick inserted in center comes out with few crumbs attached, about 30 minutes. Let cakes cool completely in pans, about 1 hour. (Cooled cakes can be wrapped in plastic wrap and stored at room temperature for up to 2 days.)

Step 4

FOR THE FROSTING: Heat brown sugar, ½ cup evaporated milk, 4 tablespoons butter, and salt in large saucepan over medium heat until mixture registers 235 degrees, 4 to 6 minutes, stirring occasionally.

Step 5

Transfer brown sugar mixture to clean bowl of stand mixer. Fit mixer with whisk attachment, add remaining ½ cup evaporated milk and remaining 4 tablespoons butter, and mix on low speed until incorporated, about 30 seconds. Let mixture cool to 130 degrees, 10 to 15 minutes.

Step 6

Add chocolate and vanilla to warm brown sugar mixture and mix on low speed until smooth, about 1 minute. Add confectioners’ sugar and continue to mix until fully incorporated and mixture is smooth, about 2 minutes longer, scraping down bowl as needed. Let frosting cool to 80 degrees, about 1 hour. (Leave whisk attachment in place. Crust will form on top of frosting; this is OK.)

Step 7

Mix frosting on medium-high speed until smooth, about 1 minute. Place 1 cake layer on serving platter. Spread 1 cup frosting evenly over top, right to edge of cake. Top with second cake layer. Spread remaining frosting evenly over top and sides of cake. Refrigerate cake until frosting is set, about 1 hour. Serve. (Cake can be refrigerated, covered, for 2 days.)