In a large mixing bowl, cream together butter and sugar on medium speed until fluffy. Add sour cream, egg, and vanilla extract. Beat until well combined.
In a separate bowl, stir together flour, cocoa powder, baking powder, baking soda and kosher salt. With mixer on low speed, gradually add dry ingredients into creamed mixture, blending until combined. Chill dough in refrigerator for 30 minutes.
Place rack in center of oven and preheat oven to 350° F. Drain cherries, reserving the juice and lightly pat cherries dry. Once dough has chilled, shape dough into 1-inch balls and place 2 inches apart on an ungreased cookie sheet. With thumb, carefully press down center of each dough ball to create a small “well.”
Make frosting and bake cookies: In small saucepan over low heat, combine chocolate pieces and sweetened condensed milk, stirring constantly until the chocolate nearly melts. Remove from heat, then stir in 4 teaspoons of reserved cherry juice, VIA and vanilla until mixture smooth and glossy. Spoon one teaspoon of frosting into the "well" you created. Press a cherry on top, and top with more frosting if desired. Bake for 8 minutes, let cool on the baking sheet for 2 minutes, then transfer cookies to a wire rack to cool completely.