Avoid butter in puff pastry dough from melting while baking in the oven
There are a few things that may be wrong here:


Oven not pre-heated enough: how long you need to preheat depends on the oven, but for 200 degrees C I'd give it 20 minutes. If your oven has not pre-heated enough it will be at too low a temperature the butter and will melt rather than turn to steam, which is what gives you lift
Oven at too low a temperature: often the temperature you set on an oven and the actual temperature is different, sometimes these differences can be really big, I've seen 35C (70F) difference in the past. It may be worth investing in an over thermometer to see whether your temperature is right. An oven thermometer will also tell you whether your oven has pre-heated as well
Dough too warm: it's important to keep your dough cold throughout the process, if butter gets too warm you will lose the lamination (layering) in the dough. When it comes to baking it if your butter is too warm it may melt, so you could try chilling your pastry in the fridge. How long to chill them depends on the size and the thickness of each pastry, the important thing is that the thickest part of the pastry stabilizes at the refrigerators's temperature. That could take 10 minutes, it could take an hour or more. For the cookies you describe try at least 20-30 minutes, or up to an hour if you have the time.