Place coconut milk, cinnamon stick (cut in half), and lemon zest in a saucepan, and bring to the boil.
Remove from heat and let it sit for 1 hour.
With an electric mixer, beat the egg yolks and powdered erythritol until the mixture becomes pale, thick, and fluffy.
Add in coconut flour and beat until all lumps have disappeared.
When the coconut milk mixture has been sitting for an hour, bring it back to the boil. Next, pour it though a sieve into a bowl.
First, add only 2 tbsp. of the warm coconut mixture to the eggs and stir to incorporate. Then, add the rest of the coconut mixture and mix.
Return the mixture to the saucepan on low heat. This step will require patience. Keep the heat low, stir occasionally until the mixture gets thick and coats the back of a spatula or a wooden spoon. This might take even 10 minutes. Do not turn the heat up as you will likely cook the eggs, and make a lumpy custard.
Pour into ramekins. When cooled, place in the fridge for at least 4 hours (overnight is best).
Before serving, sprinkle 1 tbsp. of erythritol over each Crema Catalana, making sure that the whole surface is covered. Using a blowtorch, try to caramelise the erythritol. This will not be as successful as when using ordinary sugar, but erythritol still does a fine job.
Optional: You can make Crema Catalana ice cream by simply placing the ramekins into a freezer. Stir the custard every now and then, so it does not crystallize too much. You can still top the frozen ones with “caramelised” erythritol before serving.