1. Start this pastéis de nata recipe by preheating the oven to 240 °C (464 °F). Bring the milk to a simmer in a saucepan, then remove it from the heat.
2. Roll out one puff pastry sheet on a work surface, then place the second sheet on top. Roll them into a long log, then cut it into 1 cm (0.4 inch) thick slices. Place these slices in the bottom of buttered muffin molds and press with your fingers to line the bottom and sides of the molds.
3. Return the milk to the saucepan and add the flour. Whisk to incorporate it and bring it to a simmer again while whisking. Remove from the heat as soon as it starts to boil.
4. In another saucepan, heat the sugar with a little water. A syrup will form, once it reaches a temperature of 110 °C (230 °F), immediately remove it from the heat and slowly pour it into the milk while whisking. Add the whole egg, the 2 egg yolks, and the coffee flavoring. Whisk to incorporate them.
5. Fill the puff pastry bottoms about 3/4 of the way up and bake for 10 minutes, keeping an eye on the tarts to make sure they don't get too dark. Remove from the oven and serve while still warm.