In a small saucepan, whisk together cream and espresso powder. Cook over medium-low heat, whisking frequently, just until bubbles form around sides of pan. (Do not boil.)
In a large heatproof bowl, combine chocolate and hot cream mixture; let stand for 2 minutes. Using a rubber spatula, stir until smooth and well combined. Stir in butter and vanilla until melted and well combined. Let cool for 30 minutes. Cover and refrigerate until cold, at least 3 hours.
In the bowl of a stand mixer fitted with the whisk attachment, beat ganache at medium speed until light and fluffy, 2 to 3 minutes. Use immediately.
Recipe by Bake from Scratch at https://bakefromscratch.com/brooklyn-blackout-cake/