In a microwave-safe bowl, heat the chocolate in the microwave, stirring every 20 seconds, until melted and !smooth, 1 to 3 minutes. Whisk in the cocoa until dissolved. Set aside.

Meanwhile, in a small saucepan, combine together the corn syrup, sugar, cream, milk, and salt. Place over medium heat and bring to a simmer, stirring frequently, until thickened, about 3 minutes.

Remove from heat, whisk in the butter and vanilla. Cool the mixture slightly, about 2 minutes. Then whisk in the melted chocolate mixture until smooth. Set aside to cool slightly before using.

Adjust oven rack to middle position. Preheat oven to 350F/180F. Line 2 cupcake pans with 16 cupcake liners.

In the bowl of a stand mixer fitted with the whisk attachment (or in a large bowl, if using a handheld mixer), whisk the butter on high speed until smooth.

Add the brown sugar and mix together until slightly lightened in color, about 3 minutes.

Scrape down the bowl, then add the eggs, one at a time, mixing well after each addition.

Add in the vanilla and continue mixing until lightened more in color and texture and looks fluffy, 3 to 5 minutes. Scrape down the bowl.

Add the cocoa powder, baking soda, salt and mix until well blended. Scrape down the bowl.

With the mixer on lowest speed, add one-third of the flour, followed by half of the sour cream, then mix to blend. Then add the second third of the flour, followed by the rest of the sour cream, mix again. End with the remaining flour and mix just until combined. Scrape the bowl as needed.

With the mixer running on medium low speed, drizzle in the hot water (or coffee) and mix until very smooth, about 1 minute. Batter will be very thin and soupy.

Portion the batter evenly among the cupcake liners (I like to use a 3 tablespoon measure ice cream scoop) until 2/3 full.

Bake until the tops spring back to the touch and a toothpick inserted into the center of the cupcake comes clean or with a few cooked crumbs attached, 16 to 17 minutes. Start checking after 15 minutes. The cupcakes might deflate slightly after they come out of the oven, but should NOT look sunken.

If the do sink, that means they were underbaked. They should have slightly domed tops.

Let cool in the pan for about 5 minutes or until cool enough to handle, then transfer to a wire rack to cool completely before filling and frosting.

While the cupcakes are baking and cooling, make the frosting (instructions below)

Beat the butter, peanut butter, cream, vanilla, and salt together in a large bowl with an electric mixer on medium-high speed until smooth, 1 to 2 minutes.

Reduce the mixer speed to medium-low, slowly add the confectioners' sugar, and neat until the frosting is light and fluffy, 5 to 10 minutes.

To make a hole in the cupcake, use a pairing knife to make a deep circular (1-inch in diameter) cut in the center of the cupcake. Make sure you only cut about halfway through the cupcake, and don't go all the way through the bottom. An apple corer would also work perfectly here. Using the tip of the knife, lift the cut piece of cake from deep inside and remove it completely. Let's call this the 'plug.' Cut the bottom half of the 'plug' and snack on it or do whatever you want with it. Set aside the top half of the plug, which will be used to cover the cupcake after filling them.

Using a small spoon or a piping bag or a zipper lock bag with the corner snipped off, pour in the Hot Fudge Sauce into each cupcake. Be generous:) Fill almost to the top. You might need to rewarm the sauce just slightly for easy filling.

Use the cake 'plug' to cover the tops of the cupcakes, where they have been filled.

To decorate the cupcake rim with sprinkles, use a clean paint brush to paint around the rim with melted chocolate, alternatively dipping the cupcakes in a small bowl of melted chocolate will also do the trick.

Dip the now-wet rim of the cupcake into a small bowl full of sprinkles, rolling it around to adhere to the entire circumference.

Using your favorite piping tip (I used a large round tip Ateco 808) or an offset spatula to frost the cupcakes with the Peanut Butter Frosting. The way it's done here, is by starting to pipe in the center, going aroung once, the ending in the center with a burst of pressure.

If desired, top with chocolate-covered pretzel. Serve cupcakes right away or store in an airtight container.