Step 1
In a medium bowl, whisk together 1¼ cups (156 g) all-purpose flour, 1 tsp. baking powder, and ¼ tsp. kosher salt.
Step 2
In a large bowl, using a hand mixer, beat 1 stick (113 g) unsalted butter, melted and cooled completely, and ⅔ cup (133 g) granulated sugar on medium-high speed until well combined, about 1 minute (remember the butter is melted so this will be looser than creamed softened butter and sugar).
Step 3
Add ½ cup (124 g) ube halaya, at room temperature, and 1 Tbsp. (7 g) malted milk powder and beat until well combined, about 30 seconds. Add 1 large egg, at room temperature, and 1 tsp. ube paste and beat until well combined (it’s okay if it looks a little curdled).
Step 4
Add the flour mixture all at once and beat on low speed until mostly combined but not quite. Finish mixing with a rubber spatula. Cover the bowl with plastic wrap and refrigerate until firm, at least 3 hours and up to overnight.
Step 5
When you’re ready to bake, use a 1½-Tbsp. scoop to scoop mounds of the dough onto a parchment-lined sheet pan. Smash and shape each mound into a 2″ (5-cm) disk. If things get sticky, dust your hands as needed with extra flour (but don’t overdo it).
Step 6
Toss the disks in the extra sugar until coated and transfer to a dinner plate (this will take up less space in the refrigerator). Chill for 15 minutes.
Step 7
Meanwhile, preheat the oven to 350°F (180°C).
Step 8
In a small heatproof bowl, add 75 g white chocolate, finely chopped (about ⅓ cup), and microwave in 15-second increments, whisking in between, until melted and smooth. Transfer 8 chilled dough disks to the parchment-lined sheet pan (alternating three rows of 2 with two rows of 1).
Step 9
Using a small offset spatula, spread a layer of the melted white chocolate on top of the dough disks. Add a few random vertical swipes on the sides. Bake until the tops are cracked and the edges are firm, 12–13 minutes.
Step 10
Transfer the pan to a wire rack and let cool for 5 minutes. Transfer the cookies to the wire rack and let cool completely.
Step 11
Repeat with the remaining dough, using a fresh parchment-lined sheet pan and microwaving the white chocolate for another 15 seconds or so to thin it out.