Preheat The Oven & Prep Your Pans. Start by preheating the oven to 350 degrees and greasing a 13" x 18" x 1" sheet pan with butter or spray oil (or line the pan neatly with parchment paper). 

Whisk Dry Ingredients. In a large bowl, whisk gluten-free flour, sugar, baking soda, and salt until well combined. Set aside.

Mix Eggs, Sour Cream & Vanilla. In a large liquid measuring cup or small bowl, combine eggs, sour cream and vanilla. (There will be a second where this looks like a big mess--just keep whisking and it'll come together smoothly.) 

Heat Butter, Cocoa Powder & Water. In a small saucepan, melt butter, cocoa powder, and water until almost simmering. You want it hot and smooth, so it can help thin out the batter. 

Assemble The Cake Batter. Add sour cream mixture to the dry ingredients and mix to combine. (It'll be thick and a bit dry.) Pour the cocoa mixture into the batter a bit at a time, mixing to incorporate the chocolate mixture before adding the next bit. Whisk until it's smooth and even in texture.

Pour Into Prepared Pan. Pour the sheet cake batter onto your prepared sheet cake pan. Gently smooth out the surface so it's even and goes all the way to the edges of the pan. 

Bake The Cake. Pop the cake pan into the oven and bake 18-20 minutes, or JUST until a toothpick inserted into the center of the cake comes out clean, with perhaps a few small moist crumbs. The cake is thin, so you don't want to over-bake.)

Right after you remove the cake from the oven, begin the frosting. (Unlike most cakes, you want to pour the frosting on while the cake is still warm.)

Melt Butter + Milk + Cocoa. In a small saucepan, melt butter, milk and cocoa powder together over medium heat, whisking regularly until it's almost simmering/bubbling. (Don't boil it, just heat it till it's about to bubble.) 

Mix In Powdered Sugar & Vanilla. Remove the frosting from the heat and stir in powdered sugar and vanilla, whisking until there are not clumps of powdered sugar. There may be a few small air bubbles--this is totally fine, you just don't want powdered sugar lumps. If you want to mix the pecans INTO the frosting, do that now. 

Pour Frosting Over Warm Cake. Pour the fudge frosting over the still-warm sheet cake and quickly use an offset spatula to gently spread the frosting evenly over the surface. Be gentle, since the cake is pretty moist! If you want to sprinkle the cake with pecans, do that now. 

Cool At Least 20 Minutes. Let the sheet cake cool at least 20 minutes before slicing. I prefer to cool it about an hour to let the frosting set up, but some folks love when the frosting is still a bit soft and gooey.

Slice & Serve! When you're ready, slice the cake into 24-36 slices. (For 24 slices, cut the whole cake into quarters, then cut each quarter into 6 slices. For 36 slices, cut the whole cake into quarters, then cut each quarter into 9 slices.)

Store Leftovers. If you have leftovers, cover the sheet pan with a lid, plastic wrap, or foil, and store at room temperature for 1-2 days, or the refrigerator for 2-3 days (my preference).