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 eggs and eggs yolks, mixing just until combined after each addition. Blend in 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 24 paper lined muffin cups, filling each cup about 2/3 full. Bake in preheated oven until toothpick inserted into center comes out clean, about 18 - 20 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 caramel frosting, drizzle with room temperature caramel sauce (I poured the caramel sauce in a small Ziploc bag and cut a very small tip off end to drizzle) and sprinkle with coarse sea salt. Store in an airtight container.
For the salted caramel frosting:
In the bowl of an electric stand mixer fitted with paddle attachment, whip butter on medium-high speed until pale and fluffy, about 3 - 4 minutes. Add powdered sugar and blend on low speed until mixture comes together. Add caramel sauce and vanilla and whip on medium-high speed until light and fluffy about 2 minutes.
For the caramel sauce:
Gather all of your ingredients and have them nearby ready to add to the mixture as needed. In a heavy-bottomed 3 quart saucepan, heat sugar and water over moderately high heat whisking constantly to dissolve sugar. Once mixture reaches a boil, stop whisking and allow mixture to boil until it reaches a dark amber color, carefully swirling pan occasionally. Once mixture reaches a dark amber color, immediately add butter and whisk until butter has melted then immediately remove from heat. Wait 3 seconds then carefully pour in cream and immediately whisk to combine (it will bubble vigorously). Whisk until mixture is smooth. Allow caramel to cool several minutes then pour into a glass jar to cool completely and refrigerate.