Line a muffin pan with cupcake liners and preheat oven to 350F.
In a medium bowl, whisk together flour, cocoa powder, baking soda, espresso powder, and salt. Set side. In a separate bowl or measuring cup, stir together whole milk and sour cream.
In a bowl of a stand mixer fitted with a paddle attachment, beat together butter and brown sugar on high speed until smooth and homogenous. Add egg and mix until combined.
With the mixer on low, slowly add in half of the flour mixture. Mix until just incorporated. Stream in the milk and sour cream mixture and add in the remaining flour and mix until just combined.
Using a standard ice cream scoop, portion the batter evenly among the cavities in the lined muffin pan. Fill each cupcake liner about 2/3 of the way with batter, roughly one full ice cream scoop of batter.
Bake for 19 - 22 minutes, or until a toothpick inserted into the centre of the cupcakes comes out clean. Cool the cupcakes on a wire rack.
Whipped Dark Chocolate Ganache
Coarsely chop the dark chocolate and place into a bowl. Add salt.
In a saucepan, bring the heavy cream to a boil. Once mixture has boiled, pour milk mixture over the chopped chocolate and let sit for 1 minute.
Whisk the chocolate mixture until thoroughly melted and combined. Set in the refrigerator to chill, at least 3 hours.
Once chilled and ready to use, transfer the ganache to the bowl of stand mixer fitted with the whip attachment. Whip ganache to medium-stiff peaks. Transfer to a piping bag fitted with a large piping tip of choice (I like using the Wilton 4B or 6B). Pipe two rings of chocolate ganache onto the cool cupcakes.
Decorate with pug sugar cookies or any decorations of choice.
Mocha Chocolate Ganache Cupcakes with Pug Cookies by Amy of Constellation Inspiration