To make the crust, place the flour, sugar and salt in the bowl of an electric food processor. Pulse once or twice to mix, and then add the butter. Pulse until the mixture resembles a coarse meal, about 30 seconds. Add the egg, and pulse again briefly until the dough sticks together slightly. Remove the lid from the food processor and gather the dough into a ball. Divide the dough into 16 equal portions. (You may choose to weigh each piece so they are equal.) Roll the divided pieces into balls. 

Grease muffin tins with the extra butter. Using your fingers, press a portion of the dough into each muffin tin. Once the muffin tins are prepared with the dough, place the muffin tin in the refrigerator for 30-60 minutes in order to chill the dough. 

For the filling, combine the sugar, cornstarch, and salt in a medium sized bowl. Whisk in the milk, heavy cream, egg yolks, vanilla (and nutmeg, if desired.)

Heat your oven to 375°F (190°C). Remove the muffin tins with the chilled dough from the refrigerator and fill with the custard mixture. Bake for 20 minutes.

Once removed from the oven, allow the pasteles to cool for about 15 minutes in the muffin tins. Carefully remove them from the muffin tins, and allow the pasteles to cool on a wire rack before serving.