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1
Preheat the oven to 325 degrees F. Cut a piece of foil 8 by 16-inches.
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2
Butter the corners and sides of an 8-by-8-inch baking pan.
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3
Line the pan with the foil, leaving an overhang on two sides.
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4
Beat the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment on medium-high speed until light and fluffy, about 5 minutes.
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5
Reduce the speed to medium and add the egg yolk, syrup, vanilla extract and vanilla bean seeds if using.
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6
Reduce the speed to low, add the flours and salt, all at once, and beat until just combined.
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7
Add the walnuts and continue to mix until fully combined.
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8
Using your hands, drop the dough in pieces evenly into the prepared pan.
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9
Flour your hands lightly and press the dough together and into the corners of the pan.
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10
Put a piece of waxed paper on top of the dough and use something square and heavy-another pan, a pound of cold butter-to press the dough into the pan and make a smooth, even top.
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11
Chill until firm, about 30 minutes.
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12
When it is firm, use a fork to score the dough all over.
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13
Bake until deep golden brown and starting to pull away from the sides of the pan, about 45 minutes.
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14
Cool completely on a rack, about 2 hours, then wrap with plastic wrap, and let sit at room temperature overnight.
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15
Place a lightweight cutting board or a baking rack over the top of the pan.
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16
Flip and set on the counter.
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17
Gently pull the pan from the foil and the foil from the pan to lift off the pan.
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18
Gently peel the foil off the bar.
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19
Place another lightweight cutting board on top, and flip the bar right-side-up.
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20
Using a sharp, serrated knife, slice the square in half.
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21
Then slice each half into 8 bars.
