I highly reccomend making the salted caramel the night before the cupcakes so that it has time to fully cool.
In a heavy bottomed saucepan over medium heat, add the sugar and stir every 1-2 minutes or so until it begins to melt (the sugar will start to clump up before melting).
Once the sugar is completely melted and smooth (it should be an amber color), immediately remove from the heat and stir in the butter until combined (don’t be scared if it bubbles a lot).
Add in the heavy cream, bourbon, and salt and mix until fully combined and smooth. It won't want to combine at first, but just keep stirring and it will come together.
Pour the caramel into a heavy glass jar to cool to room temperature. Can be stored in the fridge for up to a month!
If you make the caramel the night before, it can sit covered at room temperature overnight so that it's a nice pourable consistency.
Preheat the oven to 350°F.
In a small bowl, whisk together the flour, cocoa powder, espresso powder, baking powder, and salt. Set aside.
In a separate large bowl, whisk together the vegetable oil and eggs.
Then whisk in the sugar and bourbon until smooth.
Gently mix in half the dry ingredients. Then add in the milk and the remaining dry ingredients and mix until just combined. The batter will be thin.
Pour about ¼ cup of batter into each cupcake liner (they should be about halfway full).
Bake at 350°F for 18-22 minutes or until a cake tester comes out clean.
Remove from pan and let cool on wire rack.
Let the chocolate cupcakes cool completely before filling and frosting.
Salted Caramel Buttercream
In a large bowl using a handheld or stand mixer fitted with a paddle attachment, cream the butter until smooth.
Add in the powdered sugar ½ cup at a time until fully combined and smooth.
Set the mixer to low speed and slowly drizzle in the salted caramel until it's fully incorporated into the buttercream. Then add in the bourbon and beat on high speed for 3-4 minutes or until the buttercream is super light and fluffy.
If needed, add in cream 1 tablespoon at a time until desired consistency is reached.
Melt the dark chocolate and let cool almost to room temperature. It should be melted but cool to the touch. I prefer to melt the chocolate in the microwave in intervals of 20 seconds, stirring each time.
Use a cupcake corer to remove the center of each cupcake, then fill with 1-2 teaspoons of cooled salted caramel.
Frost the filled cupcakes and then drizzle with melted chocolate and salted caramel, and sprinkle with a little extra flaky sea salt.