In a medium bowl, sift together the all-purpose flour, cocoa powder, and salt. If using, add the red velvet cake mix and mix until combined.
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy. This should take about 2-3 minutes.
Mix in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Add the milk a little at a time until the dough reaches your desired consistency. It should be soft but not too sticky.
Fold in the white chocolate chips.
Enjoy immediately or chill in the refrigerator for about 30 minutes for a firmer texture.