Give it a really big moon.
Or you could have it be a moon (rather than a planet) like Io orbiting a larger planet, though with that option it's liable to be tidally locked which can cause it's own issues.
The tidal forces generated by the body orbiting it will push & pull it helping to keep it warm longer.
Io is a good example of how it works, the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean moons (quoted directly from its Wikipedia page).
You might also consider having any atmosphere in subterranean pockets closer to any warmth left in the core (not really an atmosphere) which gives the opportunity for biomes like the Movile Cave.
And then there's also heat from biological activity, any sufficient quantity of microbial extremophiles (subsurface or other) can plausibly generate enough heat of their own to have a non zero impact.
What keeps earths core hot? Scientific American says.
(1) heat from when the planet formed and accreted, which has not yet
(2) frictional heating, caused by denser core material sinking to the
centre of the planet.
(3) and heat from the decay of radioactive elements.
So there's another option, more radioactive elements than other planets have, I'm not sure how you justify that though, planets are made the way planets are made & that's pretty much the same way for all of them so their make up is probably going to be pretty uniform .. baring the lighter elements stripped of by the solar wind for those that form nearer their star.
But perhaps it could be the product of the collision of two proto planets where the heavier elements had begun to sink to the core & it's a chunk from one of them from where radioactive elements had settled to, we think some asteroids with high metal content were made that way.
If you want life of some sort on your planet then a significantly higher concentrations of radioactive elements than on Earth may be problematic so you may want to pass on that option.
You could mix & match any or all of those.