Combine butter, dark brown sugar, granulated sugar, vanilla extract and salt in a large bowl. Beat with an electric mixer set at high speed until light and fluffy.
Add the cocoa, egg and baking soda to creamed mixture. Beat until blended. Stir in the flour and white chocolate.
Chill the cookie dough, covered, for 4 to 10 hours.
Preheat the oven to 325°F. Grease baking sheets.
Shape heaping tablespoonfuls into balls. Arrange several inches apart on greased baking sheets, allowing 6 cookies per sheet.
Bake for 12 minutes for a slightly chewy cookie or 14 minutes for a crisper cookie. Cool on baking sheets for 2 minutes. Remove to wire racks to cool completely. Store in airtight containers.
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