Add flour, garlic powder, and salt to a mixing bowl, then whisk until combined. Pour in chilled milk and continue whisking until smooth.
Heat vegetable oil in large heavy bottom pan to around 370 degrees F. (Keep between 360-380 degrees)
Dip mushrooms into batter one at a time, removing with a fork, repeat with remaining mushrooms.
Gently place mushrooms into heated oil using slotted spoon, in batches, careful not to overcrowd.
Deep fry until golden brown, about 4 minutes, remove and place on a paper towel lined plate. Note: fry time may vary depending on size of mushrooms. For larger ones, you may want to fry for an extra 30-60 seconds, careful not to over-fry.
Top with a sprinkle of salt and fresh parsley, then serve.