There are many good recipes out there. They are similar and different. All of them result in a relatively thin batter, almost as thin as the one you make for crepes. The differences are mostly related to how every cook adjusts the level of sweetness and how the batter is handled and/or baked.
When you read a recipe pay attention to:
The temperature of ingredients when they are blended,
The speed of blending,
The time for the batter to rest,
The temperature of the batter before it goes to the oven,
The temperature of the oven,
The time in the oven.
All these factors are interconnected. They are adjusted to personal tastes (some like it sweet), type of canelé molds (copper or silicon), ovens, etc.
There are recipe notes important for any combination of factors. For example, you need to give enough time for the flour to hydrate. In other words, if you start with room temperature ingredients, your batter needs at least 1 hour in room temperature for proper flour hydration. If you plan to bake fresh canelés for breakfast and make your batter the day before, you can use cold ingredients — the batter is going to rest refrigerated overnight anyway.
There are other notes specifically related to the type of mold used for making these pastries. Copper molds are the best for canelés, but they are very expensive. Many people prefer silicone molds for that reason. With silicone molds, it is very difficult to get the best caramelized crust canelés are famous for, but there are tips and tricks to achieve decent results. If your molds are made of silicone, read section “Can I bake them in silicone molds?” of ChefSteps canelé class.
Measure all the ingredients using scales. Get ready all the kitchen tools. You need: sauce pan, blender or whisk, bowl, strainer, seasoned molds, baking sheet lined with silicone mat (to prevent molds from sliding).
Heat the milk and sugar(s) stirring. No need to simmer or boil it. You only need to dissolve sugar. Melt butter in hot milk, also stirring, to emulsify it while it melts. Let it cool down to room temperature.
Blend egg yolks, flour, rum and milk on the lowest speed. Rest the batter for an hour in room temperature, or overnight refrigerated.
Refrigerate the molds. Melted butter applied to a cold mold will coat more evenly. Preheat the oven to 375F. Strain the batter to remove most of air bubbles. They were incorporated into the batter during the blending/whisking.
When ready to bake, place one mold on the scales, measure 75g of batter, note the level. Fill the rest of the molds to the same level. Place them on the baking sheet, spaced, and bake till they are dark brown. In my oven it takes 1 hour and 15 minutes.
It is very important to remove pastries from their molds when they are still hot. Use kitchen towel! Turn every mold upside-down and shake to release the pastry. Let them cool down completely. Enjoy!
The best copper quality/price mold I could find are sold by JB Prince. They need to be seasoned before the first use. They are seasoned the same way as cast iron. See "Seasoning and cleaning your molds" for more instructions. Important! If for some reason you need to clean your molds with soap and/or scrubbing, they have to be seasoned again.
I buy my dextrose on amazon.com. You can use 125g of sugar instead of 110g sugar and 15g of dextrose mix. Your caneles will be more sweet.