I recommend making the ganache a few hours before you make the cupcakes so that it has time to fully firm up.
Finely chop the dark chocolate and place into a large bowl.
Place the heavy cream in a microwave safe bowl and microwave for about 1 minute or until hot to the touch but not boiling.
Pour over the chopped chocolate and let sit for about two minutes.
Using a spatula, gently mix until the chocolate is fully melted and the ganache is fully combined and smooth.
Let sit at room temperature or in the fridge until is has thickened enough to hold its shape.
If your ganache is too firm, microwave for 10 seconds to soften and then stir until ganache is the desired consistency.
Chocolate Cupcakes
Preheat the oven to 350°F and line your cupcake pan with liners.
In a small bowl, whisk together the flour, sugar, cocoa powder, espresso powder, baking powder, and salt. Set aside.
In a separate large bowl, whisk together the vegetable oil, eggs, milk, and vanilla.
Gently mix in the dry ingredients 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 cupcakes cool completely before filling and frosting.
Use a cupcake corer to remove the center of each cupcake, then fill with about 2 teaspoons of thickened chocolate ganache.
Chocolate Buttercream
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.
In a large bowl using a handheld or stand mixer fitted with a paddle attachment, cream the room temperature butter and powdered sugar until smooth.
Add in the cocoa powder and mix until fully combined.
Set the mixer to low speed and slowly drizzle in the melted chocolate until it's fully incorporated into the buttercream.
Beat the frosting on high for 4-5 minutes until light and fluffy.
If the frosting in too thick, add in 1 tablespoon of cream at a time to thin it out a bit.
Frost cooled cupcakes. I used a Wilton 1M tip for these photos.
Garnish with chocolate pearls if desired!
Notes
These cupcakes will last in the fridge for up to 3 days. However, they taste best when served at room temperature! I like to let them sit at room temperature for about an hour before serving.