What is the difference between chocolate flavor cocoa powder and drinking chocolate?
Drinking Chocolate is whole chocolate which will often also contain sugar and milk Solids.
To make cocoa powder you could put the drinking chocolate in a hydrolic press. and the cocoa buter from the chocolate would separate out. and you would be left with what's called a "cake" which you could then smash and sift into cocoa powder.

although cocoa powder would usually be produced before sugar or milk solids were added to the chocolate making process. one would call the unsweetened chocolate mixture "liquor" with is effectively made in the same process nut butter is made.  in the case of chocolate if you let the nut butter like "liquor" fat mass sit to long it would set rock hard into unsweetened chocolate.

for cocoa powder you are simply removing the cocoa Butter and only leaving cocoa powder.
the Chocolate maker can then sell the cocoa Butter for much more than you payed for the powder as there is always a higher demand for cocoa butter, ether in expensive blocks of chocolate or in the case of cadburys they were selling there butter to the cometic industry and replacing it with palm oil for there own chocolate production