In a large bowl, whisk together flour, cornstarch, baking powder, salt. Set aside.
In a stand mixer -- or in a large bowl using a hand mixer -- beat together the softened butter and both sugars until creamy.
Add the ube jam, ube extract, and vanilla, beating just until combined. Then beat in the eggs one at a time.
Slowly add the dry ingredients to the wet ingredients and mix just until there is no more white flour visible. Use a silicone spatula to gently fold in one cup of the white chocolate chips; set aside the remaining ½ cup of white chocolate chips for later.
Form the dough into a round disc, wrap with plastic wrap, and refrigerate for 2 and a half hours.
After 2.5 hours of chilling the dough, preheat the oven to 350°F. Line two baking sheets with parchment paper or silicone baking mats.
Use a medium cookie scoop to scoop out the dough and form round balls. Bake at 350°F for 10 to 12 minutes, rotating and switching the pans halfway through for an even bake.
Once cookies are done, remove them from the oven and immediately press 2 to 5 white chocolate chips gently into the tops of each cookie. Allow to cool in the pans for 5 minutes, then transfer to a cooling rack to cool completely
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Notes
Makes about 48-56 cookies.Ube jam: You can find ube jam at most Asian markets, or online. The jars come in many different sizes. If you purchase a jar larger than 10 ounces needed for this cookie recipe, be sure to weigh the ube jam using a scale to measure out 10 ounces. Ube extract: You can also find ube extract at most Asian markets, or online. Chilling the dough: This is a very important step - do not bake without chilling the dough for at least 2 hours! I find that 2.5 hours in the fridge usually yields dough firm enough to bake. However, you can chill your dough for up to 3 hours. White chocolate chips: Pressing the extra white chocolate chips into the tops of each cookie after baking gives the cookies that cute look with pops of white. But this step is optional; you can add all of the chocolate chips into the batter before baking if you prefer. The taste is the same, it's just a matter of appearances :) Storage: Store in an airtight container at room temperature for 5 to 6 days. You can also freeze these cookies in freezer-safe bags or containers for up to two months.