In a small bowl, whisk the butter and sugar together. Add the vanilla and yolk and whisk until smooth and creamy.
Sprinkle the flour, baking soda and salt over the wet ingredients, and stir until just mixed. Fold in the chocolate chips.
Scoop the dough into 2-6 portions depending on how many cookies you want. They will spread a lot- so leave at least 3-4 inches between each scoop of dough.
Bake for 12-15 minutes. Do NOT overbake. They may look doughy on the inside- but they will continue to bake while they cool. Cool on the cookie sheet for 5 minutes then transfer to a wire rack.
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