Preheat the oven to 350 degrees F. Lightly butter a 9-by-13-inch baking dish; line with parchment paper, leaving an overhang on 2 sides. Butter the parchment. Whisk the flour, oats, malted milk powder, cardamom, ginger, cinnamon, cloves and salt in a large bowl.
Beat the butter and brown sugar in a large bowl with a mixer on medium-high speed until light and fluffy, 3 to 5 minutes. Add the egg and vanilla and beat until combined. Reduce the mixer speed to low; beat in the flour mixture until just incorporated. Transfer the batter to the prepared pan and spread evenly.
Bake until the top is golden brown and the edges start to pull away from the sides of the pan, 20 to 25 minutes. Remove from the oven and sprinkle with the white chocolate, then return to the oven until the chocolate softens, about 45 seconds. Remove from the oven and spread the chocolate into a smooth layer. Top with nonpareils. Transfer to a rack and let cool completely, then lift out of the pan using the parchment; cut into squares.