To make the pastry, add the flour and butter into a large bowl. With the tips of your fingers, rub the butter into the flour. Once the mixture resembles breadcrumbs, then add a little bit of the water at a time mixing until it all comes together in a ball. You may not need all of it. Flatten and refrigerate for 20 minutes.
Meanwhile, in a small bowl, mash the sweet potato. Season with a bit of salt and pepper.
In another bowl, combine the heavy whipping cream, milk, eggs and rosemary. Season with salt and pepper.
Preheat oven to 400°F.
Remove the pastry from the fridge and roll to about 1/8 inch thick. With a 3.5 inch circle cutter, cut out 12 rounds. Gently fit them into a muffin tin.
Add about 2 tsps of sweet potato to each tartlet. Using the back of a spoon, flatten the sweet potato evenly into the bottom of each tartlet. Then carefully pour in the egg and cream mixture into each tartlet making sure to leave a little space so the filling doesn't overflow. Top with crumbled goats cheese.
Place in oven and bake for 20 minutes until the pastry is golden brown. Let cool for 5 minutes and then remove and cool on a rack. Once cooled and ready to serve, add a sprig of rosemary into each tartlet.