To make the cookies, beat together the butter and sugar until light and fluffy. Beat in the egg and vanilla.
In a separate bowl, combine the flour, cocoa powder, baking soda, and salt. Mix to combine. Add the dry ingredients to the wet and mix until well combined.
Chill the dough for at least 15 minutes to make it easier to work with.
For the filling, beat together all ingredients until well combined.
To assemble cookies, take a golf ball sized chunk of dough and roll it into a ball. Flatten the dough into a disc between your hands. Place a teaspoonful of the filling into the center of the chocolate disc. Close the disc around the filing and roll into a ball. Roll the ball of dough in sugar.
Repeat with the remaining dough, spacing the cookies 1 inch apart on a parchment lined baking sheet. Bake at 350 degrees for 10-12 minutes or until the cookies seem set. Chill on the counter for 15 minutes and then chill in the fridge for another 30 minutes.
Your hands will get messy and covered in chocolate while rolling the balls, but the end result is worth it!