Pulsing together all-purpose flour, whole wheat flour, brown sugar, baking powder, and salt in a food processor until well combined. Add cold cubed unsalted butter and pulse until the mixture resembles coarse crumbs. Incorporate vanilla extract and most of the eggs until the dough holds together but isn't sticky. Chill the dough in the refrigerator for at least 30 minutes.

In a medium saucepan, combine fresh strawberries, Medjool dates, spiced rum, honey, lemon zest, lemon juice, and your choice of ground spices. Stir the mixture as it simmers for about 10-15 minutes, allowing the strawberries to break down and the mixture to thicken. Blend smooth using an immersion blender.

Remove the dough and divide it into two equal pieces. Roll each piece out on a floured surface into a rectangle about 1/4-inch thick. Pipe a generous line of the cooled filling down the center of each rectangle. Fold the edges over the filling, sealing them tightly, then flatten the logs gently to form bars. Cut each log into individual bars.

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper and arrange your assembled strawberry nutrigrain bars on it. Bake for 18-22 minutes, or until the bottoms are golden brown and the tops are lightly set.

Once baked, remove the bars from the oven and let them cool on the baking sheet for about 10 minutes, then transfer them to a wire rack to cool completely. Once cooled, cut into bars and dust with powdered sugar if desired.