Preheat oven to 350° F.

In a medium saucepan over medium heat, melt the butter. Remove from heat to cool a bit.

While butter is melting and then cooling: Prepare an 8" round cake pan by first buttering the bottom and sides. Line bottom of pan with parchment paper, and then butter top of parchment paper. Add extra cocoa powder to the pan, and then tilt it about, allowing cocoa to distribute over the butter and cover the entire interior of the pan. Invert pan over the sink and tap it gently with your hand to remove any excess cocoa. Set pan aside.

Sift flour, cocoa, and salt into a medium bowl and then whisk in the sugar.

Add cocoa mixture to melted butter in saucepan. With a sturdy spatula, stir gently until halfway incorporated. Add whisked eggs and vanilla. Stir gently to bring all ingredients together. Transfer mixture to prepared cake pan.

Bake on lower oven rack for about 25 minutes, until the top is set but the center is still gooey. The top should be firm, but crack under light pressure from your finger. We bake this cake exactly 24 minutes for it to be perfectly set with a sticky/gooey interior. You may need to experiment with timing to achieve this perfect texture, as oven temperatures and baking equipment varies. Once you know how your oven reacts, you will know the perfect length to bake it going forward.

Remove pan from oven and let cool on a rack for 1 hour. Run a butter knife between the pan and the cake to loosen it. Invert onto cake plate. (See note below.)

Dust cake top with powdered sugar, if desired. Serve cold for a more firm and fudgy texture. Or serve slightly warm or at room temperature for a sticky/gooey texture. Top individual slices with fresh berries, whipped cream, and/or ice cream, if desired.