Preheat oven to 350 degrees Fahrenheit.
In a microwave-safe bowl, add chocolate chips and butter, microwave for 1 minute. Stir until the chocolate is melted and well combined with the butter.
1/2 cup semi-sweet chocolate chips, 1/4 cup butter
In the same bowl, add flour, powdered sugar, cocoa powder, milk, salt, and vanilla extract.
1/4 cup all-purpose flour, 1/4 cup powdered sugar + more for topping, 1/4 cup unsweetened cocoa powder, 1/4 cup milk, 1/4 teaspoon sea salt, 1/2 teaspoon vanilla extract
Mix until combined. Do not overmix! Pour the batter into a greased 8 ounce ramekin.
Place the chocolate truffle in the center, submerging it in the cake batter.
1 Hazelnut truffle
Bake for 20 minutes. Let cool for 10 minutes. Gently loosen the sides with a butter knife. Invert the cake onto a plate, and sprinkel extra powdered sugar on top, if desired.