I don’t mind if I do
Definition 1 of 2
Pro Tip 1/3Gentle Enthusiasm
'I don’t mind if I do' shows a polite interest without sounding overly eager. 
When offered a second helping of dessert at a friend's house: 'I don’t mind if I do, thank you!'
Pro Tip 2/3Social Courtesy
This phrase can signal that you're accepting an offer more to be polite than from a strong desire. 
Neighbor: 'Would you like to try these homemade cookies?' You: 'I don’t mind if I do.'
Pro Tip 3/3Light Humor
Sometimes used humorously to accept without any pretense of hesitation. 
At a party: 'We have extra cake, anybody?' You, with a smile: 'I don’t mind if I do!'