This actually sounds strikingly similar (in effect, if not cause) to the giant impact hypothesis of the Moon's formation. The gist of this hypothesis is that the young Earth was struck by a Mars-sized body named Theia. Most of Theia's mass was subsumed by the Earth; some of it was blown into orbit and coalesced into the Moon.
The immediate effect was that both bodies were heated to such a degree that their entire crust was turned to magma. As a result, both the Earth and Moon eventually became mostly round, differentiated bodies despite any transient deformation. (This is very much a live topic of research, though, and while the basic theory is pretty sound, the models of what happened after the impact need work. For instance, there's a model that suggests the Earth-Moon system spent some time as a torus before collapsing further into separate bodies, or that the impact actually formed two moons which eventually merged.)
Differentiation is the key. This is the process by which planets like Earth end up with an iron core and a lighter (mostly silicate) crust, and is driven by the relative densities of materials. Heavier materials settle to the bottom, while lighter materials are raised upwards by buoyancy. After the giant impact, Earth's and Theia's cores merged into a single iron core, and the same thing seems likely to happen here. Even if part of the core is exposed in the blast, it will take a long time for the planet to cool down enough to harden into a new configuration.
So to sum up: the planet's crust will be reduced to magma for a prolonged period. While it cools, the planet will reshape itself into a differentiated sphere with a core, mantle, and crust similar to what it has now. Depending on the force of the explosion, some of the material may end up in orbit, where it will also shape itself into a sphere (assuming there's enough of it), forming something akin to the Moon. (If you do this to Earth, the Moon is probably fine; it will gradually "sweep up" all the leftover debris, becoming somewhat larger.)
(Edit: thanks to not store bought dirt for reminding me of the difference between centrifugal sorting and buoyancy sorting.)