Line a baking sheet with paper towels.

In a large microwave-safe bowl, heat bourbon on high until an instant-read thermometer registers 180°F (82°C), about 2 minutes. (Alternatively, in a small heavy-bottomed saucepan, heat bourbon over medium heat until an instant-read thermometer registers 180°F [82°C].) Stir in dates; let stand for 20 minutes, stirring occasionally. Drain dates, reserving liquid for another use, if desired. Spread dates on prepared pan, and pat dry. Remove pits; chop dates into ¼-to ⅓-inch pieces.

Preheat oven to 350°F (180°C). Line a 13×9-inch baking pan with parchment paper, letting excess extend over sides of pan.

In a large bowl, whisk together brown sugar, melted butter, and granulated sugar until well combined. Beat in eggs and vanilla until well combined.

In another large bowl, whisk together 1¼ cups (156 grams) flour, baking powder, and salt. Gradually fold flour mixture into butter mixture just until combined.

In a medium bowl, stir together dates, pecans, and walnuts; add remaining ¼ cup (31 grams) flour, stirring until well combined. Fold date mixture into batter. Spoon into prepared pan, smoothing top.

Bake for 10 minutes. Reduce heat to 300°F (150°C); bake until a wooden pick inserted in center comes out clean and an instant-read thermometer inserted in center registers 205°F (96°C), about 40 minutes more. Let cool completely in pan. Using excess parchment as handles, remove from pan. Using a serrated knife, cut into 24 bars.