Mix the ☆ ingredients and make a strong coffee in a bowl. Soak the biscuit.
In a separate bowl, place the biscuit from Step 1 at the bottom. If you like the coffee flavor, you can add the remaining coffee from Step 1.
Put the milk and lemon zest in a sauce pan. Warm until just before it starts to boil and milk is infused with lemon zest.
Put the ingredients marked ○ into a bowl and mix well.
Add the milk from Step 3 into the mixture of Step 4, and mix well. Discard the lemon zest.
Put the mixture of Step 5 back into the sauce pan and simmer over low heat until it thickens (stir constantly with a wooden spatula to keep the natillas from burning).
When Step 6 has cooled, add the rum. Pour into the bowl of Step 2. Chill very well in the refrigerator. Serve the natillas sprinkled with cinnamon.
Story Behind this Recipe
This is one of the most popular desserts at restaurants and school lunches in Spain. Usually you place the biscuit directly at the bottom of the bowl but I soaked it in the coffee first. I wanted to make a grown-up version of natillas by adding rum to it.
If you have a cinnamon stick, heat together with the lemon zest and milk from Step 3 to make it more flavorful. Make sure you keep stirring with a wooden spoon at Step 6 until the mixture thickens. Try to keep the natillas from burning on the bottom of the sauce pan!