See the blog post above for a visual guide to these instructions.

1. Make the dough. Add the all purpose flour, whole wheat flour, brown sugar, baking powder and salt to a food processor and pulse to combine. Add in the cold cubed butter and pulse a few times until pieces are pea-sized and evenly incorporated.
2. Add in the vanilla extract and about ¾ of the whisked egg mixture. Continue pulsing until the dough comes together. To test if it's ready, press the dough between your fingers – it should stick together without crumbling (but it shouldn't be too wet and sticky). If it's still too dry, add more egg in small increments until it reaches the right consistency.
3. Shape the dough into a rectangle, then wrap dough in plastic wrap and refrigerate for at least 30 minutes or up to 2 hours.
4. Make the filling. Add all of the filling ingredients to a small saucepan and stir. Bring to a simmer, then reduce the heat to low and let it cook uncovered for about 15 minutes, until the mixture is thickened, stirring every few minutes. The liquid should be entirely evaporated and it should no longer be bubbling. It should look like a thick paste. Remove from heat and let cool slightly, then transfer to a food processor or blender and blend until mostly smooth. Transfer the mixture to a piping bag with a round tip.
5. Preheat the oven to 350˚F / 175˚C. Line a sheet pan with parchment paper and set aside.
6. Prepare the dough. On a lightly floured surface, divide the dough into four equal pieces. Roll each piece of dough into a 3"-4" long cylinder, then roll into a 9" long x 4" wide rectangle, using a bench scraper to help square up the sides.
7. Pipe in the filling. Pipe two strips of filling side by side down the middle of the dough (lengthwise), leaving a small margin on each end of the cookie so it doesn't ooze out when baking.
8. Form the cookie. Fold each side of the dough over the filling, then pinch the seam and ends closed with your fingers. Flip the log over and roll very gently with a rolling pin to flatten the cookie out and flatten the seam.
9. Bake. Carefully transfer the logs seam side down to the baking sheet. Bake for 18-22 minutes, until the bottom is lightly browned. Let cool completely and then slice each log into 4 cookies.