1-First Step: Mise en place Gather all ingredients at room temperature. Measure dry cake ingredients into a bowl: flour, sugar, cocoa, baking soda, salt. This keeps things smooth and fast.

2-Second Step: Mix cake batter Add egg, oil, buttermilk, vanilla to dry mix. Beat 2 minutes until smooth, no lumps. Batter will be thick. Pour into pan, smooth top. Bake 25 minutes or until toothpick clean. Cool fully in pan, about 1 hour. This base stays moist thanks to oil and buttermilk.

3-Third Step: Prepare mousse base Chop 12 oz chocolate, melt with 1/4 cup butter (optional from variations) over double boiler or microwave in 30-second bursts. Stir smooth, cool 10 minutes. Meanwhile, sprinkle 2 tsp gelatin over 1/4 cup cold water, let bloom 5 minutes.

4-Fourth Step: Bloom and heat gelatin Microwave bloomed gelatin 10-15 seconds until liquid, or heat gently. Stir into cooled chocolate. Whip 2 cups heavy cream with 1/4 cup powdered sugar to soft peaks, 2-3 minutes. Room-temp cream whips best.

5-Fifth Step: Fold mousse Gently fold chocolate into whipped cream in thirds. Work quick to keep air. Mixture thickens fast from gelatin. Pour over cooled cake in pan, spread even. Tap to release bubbles.

6-Sixth Step: Chill and set Refrigerate 4 hours or overnight. Mousse firms up beautifully. For ganache, heat 1/2 cup cream, pour over 4 oz chopped chocolate, stir glossy. Drizzle after unmolding.

7-Final Step: Unmold and serve Run knife around edge, release springform. Slice chilled for clean cuts. Garnish with cocoa dust, berries, or whipped cream. Pairs well with coffee. Store as below. Pro tip: Chill knife in hot water for neat slices.