Yeast Doughnuts have air pockets

Hi, 

I am trying to make doughnuts for a bakery but majority of the time we will get air pockets once fried. 

Our recipe: 

55g butter 

55g veg oil

150g egg

530g milk

21g yeast 

940g flour

170g sugar 

7g salt

Left in fridge overnight (12-15hours)

Cut then left to prove for another 45-60mins 

Fry 4 mins (large doughnuts) aboit 170-180°C

things we have changed with no better outcome:

• less milk
• Less prove time (worked for a while then back to normal)
• More prove time (worked for a while then back to normal)

Our next option to try are less yeast then maybe play with oil temp aswell. 

Any help would be much appreciated. Thanks