Preheat the oven to 350℉/180℃. Line a 6-inch/15 cm round pan with parchment paper overhanging the sides for easy cake removal.
In a bowl, add egg, granulated sugar, and vanilla extract. Whisk until combined.
Add milk and vegetable oil. Mix until combined.
Sift in glutinous rice flour, cocoa powder, baking powder, and salt. Mix until well incorporated.
Pour half of the batter into the prepared pan. Add chopped chocolate in the center. Pour the remaining batter evenly to cover the chocolate.
Tap the pan a few times to remove air bubbles. Add some chocolate as a topping.
Bake for 30-35 minutes.
Remove the cake from the pan. Best served hot or warm when the chocolate is still runny.
