Preheat your oven to 350°F (175°C) and grease or line an 8x8-inch baking pan with parchment paper.

In a mixing bowl, combine flour, cocoa powder, and powdered sugar. Add the softened butter and egg yolk, and mix until it forms a crumbly dough. Press this mixture into the bottom of your prepared pan, spreading it evenly. Bake for about 12–15 minutes until slightly firm and set.

While your crust is baking, whisk together the milk, heavy cream, sugar, and cornstarch in a saucepan over medium heat. Keep stirring until the mixture thickens and bubbles. Take it off the heat and add the vanilla and butter, stirring to incorporate. Let it cool for a few minutes.

Once your crust is cool, pour the creamy filling over it, spreading it evenly. Pop the bars back into the refrigerator for about 2 hours to set.

In a microwave-safe bowl, melt the chocolate chips and butter together until smooth. Stir until it’s glossy, then carefully pour over the set cream layer. Let it chill again for another 30 minutes.

Once everything is set up, slice your bars into squares or rectangles. Serve them cold.