Preheat oven to 350 degrees. In large heat proof mixing bowl, whisk together cocoa powder and baking soda. Carefully pour in boiling water and whisk until bubbling subsides and mixture is well blended, allow to cool for 5 minutes. Meanwhile, in a separate large mixing bowl, using an electric hand mixer set on low speed, blend together melted butter, vegetable oil, granulated sugar, vanilla and salt until well combined, about 30 seconds. Stir in egg and egg yolk, mixing just until combined after each addition. Blend in lukewarm cocoa mixture then blend in heavy cream. Slowly mix in flour and blend until combined, while scraping down sides and bottom of bowl.
Divide batter among 12 paper lined muffin cups and bake in preheated oven until toothpick inserted into center comes out clean, about 19 - 21 minutes. Remove from oven and allow to cool in muffin tin several minutes before transferring to wire rack to cool. Cool completely then frost with Chocolate Cream Cheese Frosting.
Chocolate Cream Cheese Frosting
In a large mixing bowl, with an electric hand mixer on medium speed, whip together butter and cream cheese until fluffy, about 1 minute. Stir in vanilla. Sift cocoa powder into mixture then blend until combine (you'll want to sift it to break up any lumps). Add powdered sugar and mix until smooth and fluffy.