• In a medium bowl, whisk together flour, cocoa powder, cornstarch, baking soda, and salt. Set aside.
• In a large bowl, combine melted butter, granulated sugar, brown sugar, and vanilla. Beat with an electric mixer until smooth and creamy.
• Add eggs one at a time, mixing on low speed until just combined.
• Gradually fold the dry ingredients into the wet mixture, mixing on low speed until a uniform dough forms.
• Line baking sheets with parchment paper. Roll dough into 1-inch balls and place on prepared sheets, spacing about 2 inches apart.
• Chill the dough balls for 30 minutes to help cookies hold their shape during baking.
• While dough chills, preheat the oven to 350°F (175°C).
• Bake chilled dough balls for 9–10 minutes, or until edges are set but centers remain soft. Allow cookies to cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.

Prepare chocolate topping:

• In a microwave-safe bowl, combine chocolate chips and heavy cream.
• Heat on high for 30 seconds, stir, then heat in additional 20–30 second intervals until smooth and fully melted.
• Top cookies: Spoon chocolate over the center of each cookie and gently spread toward the edges.
• Add sprinkles: Immediately decorate each cookie with Cosmic Brownie sprinkles before the chocolate sets.