Preheat oven to 350° F. Line muffin pan with paper baking cups. Cream together the butter and sugar in the work bowl of a standing mixer. Add the eggs one at a time, thoroughly incorporating each before adding the next. Add the vanilla.
In a separate bowl, whisk flour and leavening to combine. At low speed, add half the dry ingredients to the butter and egg mixture, mix to combine. Add half the sour cream, then repeat with remaining dry ingredients and sour cream, mixing to combine after each addition.
Scoop batter into prepared muffin cups, filling each cup 2/3 full. Bake for 15 minutes, or until toothpick inserted in the center comes out clean. Let cool slightly, then remove from pan and let cool completely on a cooling rack. Dip cupcake tops into chocolate glaze and decorate with pearl sugar, dragées, or flakes of fleur de sel.
Melt chocolate in a bowl set over simmering water. When completely melted, remove bowl from water and let sit 5 minutes. Slowly whisk butter pieces and powdered sugar into the melted chocolate until you have a smooth consistency.