In the bowl of a stand mixer fitted with the paddle (or a large bowl with an electric mixer), beat the softened butter, granulated sugar, and light brown sugar on medium-high until creamed and fluffy, about 3 to 4 minutes.
Scrape the bowl, then add the egg and vanilla and beat on medium-high until incorporated, about 2 minutes.
Scrape the bowl again, add 2 tablespoons cream or half-and-half, and beat on medium-high until combined, about 1 minute; the batter may look slightly grainy, which is fine.
Scrape the bowl, add the flour, cocoa powder, baking soda, and salt, and beat on low until just combined, about 1 minute.
Scrape the bowl, add 2 cups semi-sweet chocolate chips, and mix on low until evenly distributed, about 30 seconds.
Using a 2-tablespoon cookie scoop or your hands, form about 19–20 equal mounds of dough. Place the mounds on a large plate or tray, cover with plastic wrap, and refrigerate for at least 3 hours (or up to 5 days). Do not bake unchilled dough.
Preheat the oven to 350°F (175°C). Line a baking sheet with a Silpat, parchment, or spray with cooking spray.
Flatten each chilled dough mound about halfway, place on the prepared baking sheet at least 2 inches apart (about 8 per sheet), and bake 8–10 minutes until the edges are set and the tops are just set and slightly glossy.
Let the cookies cool completely on the baking sheet; they will firm up as they cool.
Make the ganache: place the 1 1/3 cups (about 8 oz) semi-sweet chocolate chips in a medium bowl.
Heat 1/2 cup cream or half-and-half in a microwave-safe bowl on high until just boiling (about 30 seconds, watch closely).
Pour the hot cream over the chocolate chips, cover to trap heat, and let sit 10 minutes. Then whisk until smooth, microwaving 15–30 seconds if needed to help melt any stubborn chips.
Spoon about 1–2 tablespoons of ganache onto the top of each cooled cookie and spread lightly if desired.
Immediately sprinkle each cookie with sprinkles as desired.