Preheat the oven to 350°F (175°C). Lightly spray an 8-inch square baking pan with cooking spray.

In a small bowl, whisk together ½ cup sugar and ¼ cup cocoa powder. Set aside for later use.

In a large mixing bowl, whisk together the flour, remaining ½ cup sugar, remaining ¼ cup cocoa powder, baking powder, and salt.

In a medium bowl, whisk together the milk, melted butter, egg yolk, and vanilla extract until smooth.

Stir the wet ingredients into the dry ingredients until just combined. Fold in the semisweet chocolate chips. The batter will be thick.

Spread the batter evenly into the prepared baking pan. Sprinkle the reserved cocoa and sugar mixture evenly over the top.

Gently pour the boiling water over the batter. Do not stir. This allows the sauce to form underneath the cake as it bakes.

Bake for 22-24 minutes, until the top is cracked, the sauce is bubbling, and a toothpick inserted into the cake comes out mostly clean.

Let the cake cool on a wire rack for 10 minutes before serving. Serve warm with vanilla ice cream for the best experience.