Combine the chocolate and heavy cream – Place the 120 g of dark chocolate and 200 g of heavy cream together in a small saucepan. Break the chocolate into smaller pieces if it's in a large block — this helps it melt faster and more evenly.

Melt over low heat – Turn the heat to low and warm the mixture, stirring constantly with a spatula or a whisk. Do not walk away from the pot. Chocolate can burn very easily, and burnt chocolate cannot be saved. Keep stirring the entire time.

Add the butter – Once the chocolate and cream are mostly melted and starting to come together, add the 30 g of unsalted butter to the pan.

Stir until completely smooth – Continue stirring over low heat until the butter is fully melted and the entire mixture is completely smooth, glossy, shiny, and well combined. There should be no lumps, no streaks, and no grainy texture. The mixture should look like liquid chocolate velvet.

Remove from heat immediately – Take the saucepan off the heat as soon as the mixture is smooth and all ingredients are fully incorporated. Overheating can cause the chocolate to seize (become grainy and lumpy) or the fats to separate from the liquids. As soon as it's smooth, it's done.

Cool the mixture to warm – Let the chocolate mixture sit at room temperature until it is warm but no longer hot to the touch. This step is important. If you pour hot chocolate mixture into cold molds, condensation will form inside the molds, which creates unwanted ice crystals. Let it cool until it feels about body temperature.

Transfer to a piping bag – Pour the warm chocolate mixture into a piping bag. This makes it much easier to fill the molds neatly without making a mess or spilling. If you don't have a piping bag, you can use a spoon or a small measuring cup with a pouring spout. A piping bag just gives you more control.

Fill the popsicle molds – Carefully pipe or pour the warm chocolate mixture into your popsicle molds. Fill each mold almost to the top. Insert the popsicle sticks into each mold.

Freeze for at least 4 hours – Place the filled molds carefully in the freezer, making sure they are standing upright and won't tip over. Freeze for a minimum of 4 hours, or until the popsicles are completely solid from center to edge. For best results, freeze overnight. The longer they freeze, the more stable they become.