A little butter leaking out around the corners of your baked pastry is perfectly normal. However, a big pool of butter is not. Your butter could have been too warm when laminating. If your dough feels warm while laminating, stop and put the dough in the fridge for 30 minutes before proceeding. Or your pastry wasn't cold enough before going into the oven. Always make sure to refrigerate it for at least 30 minutes or freeze it for 15 before putting it into the hot oven.