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Cream butter and brown sugar until fluffy. Beat in egg yolk and vanilla.
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Add flour, mixing well. (This is very sticky dough).
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Spread dough in a greased 9x13-inch baking pan. Bake in a 350° pre-heated oven for 18 to 20 minutes.
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Remove from oven and immediately sprinkle with the chocolate chips.
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Wait about five minutes then spread the chocolate over the top evenly.
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Sprinkle with nuts. Cut into squares when cool.