Step 1: In a medium saucepan, combine the granulated sugar, unsweetened cocoa powder, and a pinch of salt. Whisk these dry ingredients together thoroughly to break up any lumps.
Step 2: Gradually whisk in the whole milk (or half-and-half) until the mixture is completely smooth. Then, whisk in the sweetened condensed milk and vanilla extract.
Step 3: Place the saucepan over medium heat. Cook, stirring constantly with a whisk, until the mixture just comes to a gentle simmer. Do not boil vigorously; just heat until the sugar is fully dissolved and the mixture is slightly thickened (about 3-5 minutes).
Step 4: Remove from heat. For an extra smooth texture, pour the mixture through a fine-mesh sieve into a clean bowl.
Step 5: Let the fudgesicle mixture cool at room temperature for 15-20 minutes, then cover and refrigerate for at least 2 hours, or until thoroughly chilled. This step is crucial for preventing ice crystals.
Step 6: Once chilled, stir the mixture again. Pour the liquid into your popsicle molds, leaving about 1/4 inch of space at the top. Insert popsicle sticks.
Step 7: Freeze for at least 6-8 hours, or preferably overnight, until the fudgesicles are completely solid.
Step 8: To unmold, briefly run the molds under warm water (10-15 seconds) to loosen the edges. Gently pull on the stick to release. Serve immediately. Store leftover fudgesicles in an airtight container in the freezer for up to 2-3 weeks.