In the heatproof bowl of a stand mixer, whisk together eggs, sugar, and vanilla by hand. Place bowl over a saucepan of simmering water. Cook, whisking occasionally, until an instant-read thermometer registers 110°F (43°C).

Carefully return bowl to stand mixer. Using the whisk attachment, beat at high speed until thick, tripled in volume, and ribbon-consistency, 3 to 4 minutes.

In a small bowl, sift together flour, 3 tablespoons (15 grams) cocoa, and salt. Using a large balloon whisk, gently fold flour mixture into egg mixture in two additions just until combined.

Transfer 1 cup (112 grams) batter to another small bowl; whisk in oil until combined. Fold oil mixture into batter in two additions just until combined. Pour batter into prepared pan, and smooth flat with an offset spatula using as few strokes as possible.

Bake until top looks dry and cake springs back when lightly pressed, 8 to 10 minutes. Immediately run a thin knife or a small offset spatula around edges of cake. Using a fine-mesh sieve, dust remaining 2 tablespoons (10 grams) cocoa all over cake. Invert cake onto a large cutting board covered with a clean kitchen towel. Carefully remove hot pan and parchment.

In a small saucepan, bring milk, sugar, and cocoa to a boil over medium heat, whisking frequently until mixture is smooth. Remove from heat; whisk in vanilla. Let cool completely before using.

In the heatproof bowl of a stand mixer, whisk together egg whites, sugar, salt, and cream of tartar by hand. Place bowl over a saucepan of simmering water. Cook, whisking constantly, until sugar dissolves and an instant-read thermometer registers 160°F (71°C).

Carefully return bowl to stand mixer. Using the whisk attachment, beat at high speed until white and fluffy, 5 to 6 minutes.

Add butter, one piece at a time, beating until combined after each addition. (Mixture may look curdled at this point, but it will come together.) Beat at high speed for 3 minutes. Beat in vanilla. Use immediately, or refrigerate in an airtight container for up to 5 days. If refrigerated, let come to room temperature and re-whip in stand mixer until fluffy before using.

In a medium heatproof bowl, place chocolate.

In a medium saucepan, heat cream and butter over medium heat, whisking frequently, just until bubbles form around sides of pan. (Do not boil.) Remove from heat; pour onto chocolate. Let stand for 5 minutes; whisk until chocolate is melted and mixture is smooth. Use immediately.