Pumpkin is diving into hardcore sex on her OnlyFans account ... and Honey Boo Boo's sister tells TMZ there's enough dessert for seconds and thirds.

Here's the deal ... Pumpkin -- real name Lauryn Efird -- joined the adult content platform in March ... and, at the time, she said she would start with some naughty nudes before going to pound town ... and that day has finally arrived.

Lauryn-Mychelle-Shannon-Darrin-Kitchens-sub-fb-1 Pumpkin tells TMZ ... "I really started doing hardcore because, truthfully, it's another revenue of money and so many options to do and give."

In one video for sale on her OF, Pumpkin is smashing her boyfriend, Darrin Kitchens.

The video has been available as an additional high-dollar purchase for subscribers -- and it's exactly what you'd imagine ... they're banging on a bed in front of a camera.