Translation:The cook always uses this technique.
I'd say that when in doubt, use the den/det här construct. Even though both ways are equally valid, some sentences (like the one in this example) can sound slightly off - too technical, for instance - to a native when using denna/detta. But there's absolutely nothing wrong with it.
Also note that the mixed version denna tekniken is very common in everyday speech, particularly amongst youths, and will likely be equally accepted at some point in the future. For the time being, however, it's still considered ungrammatical.