Cosmochemistry within the solar system is mostly performed through analyzing the chemical compositions of meteorites, which can shed light upon the formation and evolution of the solar system. We have a program that uniquely measures the chemical compositions of extrasolar rocky objects, which brings us into a new area of extrasolar cosmochemistry. Specifically, we perform high-resolution spectroscopic observations on some white dwarfs, whose atmospheres were enriched with heavy elements by accreting from their own asteroids. Highlights of the field includes: (i) to zeroth order, the compositions of extrasolar planetesimals resemble rocky solar system objects. In almost all cases, O, Mg, Si and Fe make up more than 85% of the total mass. (ii) There is strong evidence that extrasolar asteroids have gone through additional processing, such as differentiation, collisions and meltings. Looking into the future, we have initiated a pilot search for evidence of Earth-analog plate tectonics in extrasolar planetesimals.