Preparation

Make the streusel

• In a medium bowl, whisk the oats, sugar, flour, cinnamon, nutmeg, and salt. Add the butter; using your fingertips, smash and rub the butter into the dry mix until it disappears into a clumpy meal. Fold in the pecans and white chocolate. Use immediately, or transfer to an airtight container and refrigerate until needed, up to 1 week.

Make the bars

• Position a rack in the center of the oven, and heat the oven to 350°F. Line a 9×13-inch cake pan with foil so that the foil overhangs the short ends by a couple of inches. In the bowl of a food processor, grind the flour and white chocolate until powdery and fine, with no visible chunks of white chocolate.
• Meanwhile, in the bowl of a stand mixer fitted with the paddle attachment, combine the butter, sugar, malted milk powder, vanilla, salt, baking powder, and nutmeg. Mix on low speed to moisten, about 1 minute, then increase to medium and beat until very soft, pale, and fluffy, about 6 minutes, pausing to scrape the bowl with a flexible spatula as needed.
• 混合运行,加入鸡蛋,继续beating until smooth. Reduce the speed to low, and add the white chocolate-flour mixture in three additions, alternating with the milk in two additions, until the mixture forms a smooth, thick batter. Transfer to the prepared pan, spread into an even layer, and sprinkle the streusel on top. Bake until the white chocolate and pecan pieces are lightly browned and the bar cookie looks puffed and firm around the edges, about 35 minutes. Cool to room temperature, then use the foil liner handles to lift the entire bar from the pan. Using a sharp knife, cut into 2-1/4-inch squares. Store in an airtight container for up to 4 days at room temperature.