Prepare inner layer / custard: Combine all inner layer / custard ingredients in a small saucepan over medium heat. Whisk well to combine. Bring to a simmer. Simmer, whisking frequently, until the mixture visibly thickens, 2 to 4 minutes. Set aside.
Prepare outer layer / shell: Combine coconut milk, rice, rice flour, sugar and salt in a blender. Blend until smooth. Add mineral water and pulse blender once or twice just until incorporated. (The batter should be a smooth consistency that pours easily, similar to pancake batter. If it is too thick to easily pour, add more mineral water a bit at a time.)
Heat an Ebelskiver or Takoyaki pan over medium heat. When the pan is hot, brush the inside of the holes with coconut oil (you can melt the oil first and do this with a brush or use a paper towel to carefully rub oil on the insides).
Pour shell batter into each of the holes, leaving about ¼ inch of headspace at the top. Then quickly use a 1 teaspoon measuring spoon to spoon the custard batter into the center.
Let the pancakes cook until golden brown on the bottom and still soft in the center, 5 to 7 minutes. (Note: You may need to adjust the heat of the pan and / or the cook time as you go to be sure that the pancakes get an even, deep golden brown color on the outside.)
If adding toppings, sprinkle them on top in the last 2 minutes of cooking.
When the pancakes are done, use a spoon to gently remove them from the pan and transfer them to a plate or cooling rack.
Repeat the process until all of the batter has been used.
Allow to cool slightly (they are piping hot right after they are cooked), but serve while still warm to retain the crisp shell and soft centers.