Place an oven rack in the center of the oven and preheat to 350 F (180 C). Grease and flour a 10-inch/25.4 cm bundt pan and reserve.
To make the cake batter: In a small, stainless steel saucepan, combine milk and cocoa powder together and bring to a boil over medium-high heat, whisking frequently. Remove from heat and let come to room temperature. In a mixer fitted with the whisk attachment, mix together the sugar, salt, baking soda, eggs, and egg yolk on low speed for about 60 seconds. Add the buttermilk, oil, vanilla extract, and vanilla bean seeds (if desired), mixing on low again just until combined. Add the flour and mix on low for about 15 seconds; then, increase speed to medium and mix for about 45-60 seconds. Add the cooled cocoa mixture and mix on medium speed just until combined. The batter will be very loose!!! Pour batter into the prepared pan and bake for 50-55 minutes, or until a toothpick inserted into the cake comes out clean.
Let the Easter chocolate cake cool completely in the pan over a rack, and invert onto a serving plate.
To make the icing: Place the chopped chocolate in a heatproof bowl or a double boiler, and set the bowl over a pot of barely simmering water. Please, make sure that the bottom of the bowl does not touch the boiling water!!! When the chocolate has melted completely, remove the bowl from the heat and whisk the melted butter and the vanilla into the melted chocolate until thoroughly incorporated. Sift in half of the powdered sugar, add the sour cream, and whisk to combine. Then, sift in the remaining powdered sugar and whisk until smooth (The glaze should be thick and shiny). Pour the glaze over the Easter chocolate cake, covering it completely, and let cool down. Decorate the Easter chocolate cake as desired and enjoy!
Storage: Easter chocolate cake should be stored at room temperature in an airtight container.