Preheat oven to 350 F.
Sift together flour, salt, baking soda, and cocoa powder into a separate medium-sized bowl. Use a fork or a whisk to ensure that the ingredients are mixed well after sifting. Set aside.
In a medium-sized mixing bowl or the bowl of a stand mixer, mix butter and sugar until fully combined. The end result should look like wet sand.
Melt the chocolate in the microwave or in a double boiler.
Once the melted chocolate is cool enough to touch, add it to the butter/sugar mixture and mix until just combined.
Mix in eggs and egg yolks one at a time until just combined.
Mix in oil, vanilla, and sour cream until just combined.
Add dry ingredients to wet ingredients in three additions, mixing until just combined after each addition.
Mix in water until just combined.
Fill cupcake liners 2/3 full and put in the oven.
Check for doneness at 18 minutes - a toothpick inserted into the center of a cupcake should come out clean. If they are not done, check again every three minutes until they are done.
Immediately remove cupcakes from the cupcake tin and place on a cooling rack or on the counter to cool to room temperature.
Cut out a cone in the center of each cupcake going almost all of the way to the bottom. Go ahead and eat the removed cones. You won't be needing them!
Fill the holes with German chocolate frosting using a spoon, scoop, or piping bag. They can mound over the top of the hole just a little bit.