In a large bowl, whisk together the heavy cream, whole milk, sweetened condensed milk, and vanilla extract until well-combined.
Divide the mixture in half (each half will be just over 1½ cups).
Add the cocoa powder to one half; whisk until well-combined. Note: At first, it will seem as though the cocoa powder isn't going to combine, but keep whisking and it will.
Pour the vanilla mixture into half of the popsicle molds and pour the chocolate into the other half.
Cover and insert the wooden ice pop sticks. Note: If your sticks happen to sink too far down (sometimes they do, sometimes they don't), just freeze the ice cream bars for an hour or two and then add the sticks. By that time, the mixture will have thickened up, and you can insert the sticks only as far as you want them.
Freeze until firm, at least 8 hours (I usually leave them in the freezer overnight).
Carefully remove from molds and serve immediately.