Coat the molds. Set 2 6-muffin molds or 1 12-muffin mold (preferably the silicone—flexible—type) in a large pan that is at least 1 1/2 inches deep (a large roasting pan works well). Scrape the thick cajeta into a microwave-safe measuring cup and microwave for 30 or 45 seconds, until soft and easily pourable. Pour 1 tablespoon cajeta into 8 of the cups, tilting the mold(s) so that the cajeta completely covers the bottom. Turn on the oven to 325 degrees and adjust a rack to the middle.
Make the custard. In a small (1-quart) saucepan set over medium heat, warm the milk until it is steaming. In a large bowl, whisk together the eggs and egg yolks, then scrape in the sweetened condensed milk, measure in the vanilla and whisk until smooth. Slowly pour in the hot milk, whisking all the time. Gently ladle or pour a portion of the custard into each of the cajeta-lined muffin molds. If you have enough custard to fill another, coat the bottom with cajeta, then pour in the remaining mixture.
Bake the flanes. Heat a couple quarts water until steaming. Open the oven door, pull out the middle rack, slide the pan onto the rack, then pour hot water around the muffin molds to a depth of about 1 inch. Carefully slide the rack into the oven. Bake until the flanes are just barely set in the middle, 35 to 40 minutes. Remove from the oven and let cool in the hot water bath, then remove and refrigerate until thoroughly chilled, about 2 hours.
Unmold and serve the flanes. Run a small knife around the edge of each of the flanes to release them from the sides of the molds. Next, flip the mold(s) over onto a rimmed baking sheet or deep platter. Here’s the easiest way I know to do that: Set the mold(s) onto a rimmed baking sheet. Flip a second rimmed baking sheet on top of them, then, holding the two baking sheets securely, flip them over. Shake the molds gently back and forth, then up and down. (Squeeze the sides of the flexible molds slightly if you need to ensure that each flan has dropped). Remove the molds. Scrape any remaining caramel out of the molds and onto the flanes, then use a small metal spatula to transfer each one to a small serving plate. Divide the caramel between them and you’re ready to serve.