How to Make It
In a large mixing bowl, beat together butter and sugar until lightened, about 3 minutes. Add egg, egg yolk, and vanilla and beat until fluffy, about 2 minutes. In another bowl, whisk together flour, baking powder, and salt. Add half of the sour cream to the butter-sugar mixture and mix to combine. Add half of the flour mixture and beat on low until incorporated. Repeat process with remaining sour cream and flour mixture. Once dough is fully combined, wrap in plastic wrap and refrigerate for at least 1 hour.
Preheat oven to 375°F. Line 2 baking sheets with parchment paper; set aside. On a well-floured surface, place the dough and dust the surface with additional flour. Roll dough out to a 10-inch wide circle that is 1/2 inch thick. Flour a round 2-inch cutter and punch out cookies from the dough, placing each circle about 2 inches apart on prepared baking sheets. Re-roll scraps of dough and cut out remaining cookie circles.
Bake until the surface is dry and the bottoms of the cookies begin to take on a little color, about 12 minutes. Remove to a cooling rack to cool completely.
In a saucepan, heat cream over medium-high until steaming. Remove from heat and add chocolate chips and vanilla. Allow to sit for 5 minutes before whisking until smooth. Stir in butter. Add powdered sugar and whisk until sugar is fully incorporated.
Dip the bottom of each cookie in the warm chocolate mixture and gently lift up, allowing the excess chocolate to drip off. Turn the cookie upside down so the chocolate is facing up and place on a cooling rack to dry. Repeat with remaining cookies. Allow chocolate to set before serving.