Both. Build it on the side: evenings and weekends on your own equipment NOT at the employer. Get undeniable traction (preferably sales and investment) before

Both. Build it on the side: evenings and weekends on your own equipment NOT at the employer. Get undeniable traction (preferably sales and investment) before working-out a resignation and continuity plan with your employer. Good employers (managers with rational egos and empathy) should want you back if it doesn't work out because you're a known quantity. Do NOT burn your ships (or bridges) behind you like Cortés.