In a microwave-safe mug, combine the flour, sugar, cocoa powder, baking powder, and salt. Mix well until all dry ingredients are evenly blended.
Add the milk, vegetable oil, and vanilla extract to the mug. Stir until the batter is smooth and well-combined.
Push the chocolate square or chocolate chips into the center of the batter gently, submerging it slightly.
Microwave the mug on high for about 40-60 seconds. Start checking at 40 seconds; the cake should be slightly firm around the edges but soft and gooey in the middle. Avoid overcooking, as the lava effect comes from the melted chocolate.
Let it cool for a minute. If desired, dust with powdered sugar before serving for an elegant touch.
Notes
For an elegant touch, dust with powdered sugar before serving.