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1
Line a small baking tray or baking tin (one that will fit inside your freezer) with baking paper.
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2
Beat the butter and sugars in the bowl of an electric mixer until combined but not too creamy - you are not aiming for light and fluffy here, as that would make the cookies too cakey. Add the vanilla and egg yolks, and mix well.
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3
Put the flour, salt and baking soda in another bowl and whisk together well. Add to the butter mixture along with the chocolate, and mix until combined.
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4
Scoop individual portions of cookie dough on to the lined baking tray or tin. If using spoons, pat each portion into a little ball. Cover with clingfilm and freeze for at least an hour, or up to a month. If you are pushed for time, or simply impatient, you can bake the cookies straight away, but they will be slightly flatter and less even than the ones here.
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5
When ready to bake, preheat your oven to 350 F. Line a large baking tray or tin with baking paper and arrange the cookies on it evenly, leaving enough space between them so they have room to expand during baking (when they will almost double in size). If you are baking from frozen, allow the cookies five to 10 minutes out of the freezer before placing in the oven.
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6
Bake for 18-20 minutes, checking constantly, until the center of each cookie is slightly soft and under-baked but the edges are crisp and golden. Remove from the oven and allow to cool on the tray for 10 minutes before serving. These cookies will keep for up to five days in an airtight container.
