Preheat oven to 350°F (175°C). Grease a 4-inch cake pan and line the bottom with parchment. Whisk flour, baking powder, and salt in a small bowl. In another bowl, cream butter and sugar until fluffy, then beat in egg, sour cream, milk, and vanilla. Fold in dry ingredients until just combined, then pour into the pan.

Bake for 18 to 22 minutes until a toothpick comes out clean. Cool in the pan 10 minutes, then transfer to a rack to cool completely.

Melt dark chocolate and white chocolate separately in the microwave in 15-second bursts, stirring each time until smooth. Transfer each to a piping bag or zip-top bag.

Pipe the panda face onto the cooled cake using dark chocolate for ears, eye patches, nose, and mouth. Add white chocolate dots for eye highlights. Let set 10 minutes before serving.