Preheat oven to 350 degrees.
Whisk together flour, cocoa powder, baking soda and salt. Set aside.
Cream softened butter and granulated sugar for about two minutes. Scrape the bowl.
Add the egg and vanilla. Beat for two minutes.
Add the sour cream and half of the flour/cocoa mixture. Mix until moistened.
Add the milk and the second half of the flour/cocoa mixture. Stir until just combined.
Divide the batter evenly between twelve lined muffin cups.
Bake at 350 degrees for 18-20 minutes.
Remove the cupcakes from the oven, immediately scoop out the middle and fill each of the holes with one of your homemade chocolate truffles. The truffles will immediately start to melt. You can return them to the still-warm oven for a couple of minutes to melt the truffles even more so that they completely fill in the hole, but it’s totally optional. Do not attempt to remove the cupcakes from the muffin pan while the cupcakes are still warm. Move the cupcakes, still in the muffin pan, to the fridge to allow the chocolate truffle center to set.
Remove from the fridge, frost with chocolate buttercream frosting (recipe below) and garnish with chocolate curls.