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1
In a bowl, mix the flour, sugar, egg yolks, and butter. Knead by hand until a ball is formed. If you have a robot, place this mixture in the bowl with the robot fitted with the hook. Divide the dough in two equal portions, and place in two separate bowls.
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2
In one bowl, add the cocoa, and mix with dough, forming a ball again.
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3
In the other bowl, grate half a tonka bean (if you can't find one, leave the dough plain or stir in the seeds of a vanilla bean), mix well, and form the other ball.
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4
Wrap the two dough balls separately in plastic wrap, and refrigerate for at least 1 hour.
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5
Remove from the refrigerator and knead the balls separately to loosen up the dough a little bit.
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6
Spread the cocoa dough on a floured surface, and right next to it, spread out the tonka bean dough. Overlap the two and roll them together. Wrap the dough strand tightly with a plastic wrap. Refrigerate for about 30 minutes.
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7
Preheat oven to 400 degrees Fahrenheit.
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8
Remove the dough strand from the plastic wrap using a knife, cut into 3 to 4 millimeter slices. Arrange the slices on a baking sheet lined with parchment paper.
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9
Bake for about 10 minutes.
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10
Cool on a rack.
