1. Prepare Oven and Pan:

• Preheat your oven to 350°F (175°C).
• Create a parchment paper sling for your 8×8-inch square baking pan: Cut two 12-inch lengths of parchment paper. Trim each to fit the 8-inch width of the pan. Press one sheet into the bottom and up the sides of the pan. Place the second sheet perpendicular to the first, covering the other two sides. This will make it very easy to lift the entire block of bars out later.

2. Make the Shortbread Crust:

• Food Processor Method: In the bowl of a food processor, combine the 1 cup of all-purpose flour, ¼ cup granulated sugar, and ¼ teaspoon salt. Pulse briefly to combine. Add the chunks of cold butter to the bowl. Run the machine continuously for 30 seconds to 1 minute, until the mixture clumps together and forms a soft dough.
• (Alternative No-Food Processor Method): If you don’t have a food processor, ensure your butter is at room temperature. Beat the butter and sugar together until creamy. Mix in the flour and salt until just combined. Chill the soft dough for about 15 minutes to make it easier to handle.
• Transfer the dough to your prepared baking pan. Press it evenly and firmly across the bottom and about ¼-inch up the sides of the pan.
• Par-Bake the Crust: Bake for 15 minutes, until the crust is very pale golden. For speed, the recipe suggests transferring the hot pan to a cooling rack in your freezer for 10 to 15 minutes while you prepare the filling.

3. Make the Almond Frangipane Filling:

• In your food processor bowl (no need to clean it after making the crust), combine the ¾ cup of toasted sliced almonds, 6 tablespoons granulated sugar, 1 tablespoon all-purpose flour, and a few pinches of sea salt. Grind these together until the almonds are powdery.
• Add the chunks of cold butter to the machine. Run the machine until the butter is fully incorporated and no large bits are visible.
• Add the large egg and any optional flavorings (almond extract, brandy). Blend just until the filling is combined into a smooth paste.

4. Assemble the Layers:

• Remove the mostly cooled crust from the freezer. It’s okay if it’s still slightly warm.
• Spread the prepared almond frangipane filling evenly over the par-baked crust.

5. Arrange the Rhubarb Topping:

• Trim your fresh rhubarb stalks. The recipe suggests cutting them in half horizontally (like splitting a hamburger bun) to get flatter pieces.
• Keeping the top (pinker) and bottom (greener) pieces matched, cut the stalks on a diagonal into about 1 ¼-inch lengths. This creates “V” shaped pieces.
• Arrange the rhubarb pieces, color side up, over the frangipane filling. For the ombré chevron pattern described, start with the greener segments at one end and gradually work your way to the pinker segments at the other end. (Or, as the author notes, simply scatter the pieces all over!).
• Sprinkle the 1 teaspoon of granulated sugar evenly over the fruit.

6. Bake the Bars:

• Place the pan on the middle rack of the preheated 350°F oven.
• Bake for 45 to 55 minutes. The bars are done when the frangipane filling is golden on top and a toothpick inserted into the almond cream (avoiding rhubarb) comes out clean. The rhubarb pattern may shift slightly as the filling puffs up.

7. Cool and Finish:

• Let the bars cool completely in the pan on a wire rack. For the cleanest cuts, it is best to chill the bars thoroughly in the refrigerator or even briefly in the freezer.
• Once cooled/chilled, use the parchment paper sling to lift the entire block out of the pan onto a cutting board.
• To Finish (Optional): Lightly dust the bars with powdered sugar. Alternatively, for a glossier, bakery-style finish, gently warm about ¼ cup of jam (apricot is lovely) and brush it over the top of the cooled bars.
• Using a sharp serrated knife, gently slice into 16 2×2-inch squares.

8. Serve and Store:

• Serve the Almond Rhubarb Picnic Bars chilled or at room temperature.
• While they can be stored at room temperature for a day or so, they keep longer and, as the author notes, taste wonderful when stored in an airtight container in the refrigerator.