In a bowl, combine flour, cornstarch, salt and ground pepper. Add the water and whisk together until well-blended and smooth. Stir in annatto/achuete powder to the mixture and mix well until color is evenly distributed.
Add 1 cup shrimps and grated sweet potato in the mixture and lightly stir to distribute.
Heat oil in a skillet or frying pan over medium-high heat.
Fill a small saucer evenly with about 2-3 tablespoon of the mixture and add 2-3 more shrimps on top. Gently tilt the saucer to slide the mixture into the hot oil and cook for about 3 to 4 minutes on each side or until they turn golden brown.
Transfer the deep-fried ukoy in a plate lined with paper towel to absorb the excess oil.