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We present the method and observations for the measurement of theExtragalactic Background Light (EBL) utilizing the shadowing effect of a darkcloud. We measure the surface brightness difference between the opaque cloudcore and its unobscured surroundings. In the difference the large atmosphericand Zodiacal light components are eliminated and the only remaining foregroundcomponent is the scattered starlight from the cloud itself. Although muchsmaller, its separation is the key problem in the method. For its separation weuse spectroscopy. While the scattered starlight has the characteristicFraunhofer lines and 400 nm discontinuity the EBL spectrum is smooth andwithout these features. Medium resolution spectrophotometry at $\lambda$ = 380- 580 nm was performed with ${VLT}$/FORS at ESO of the surface brightness inand around the high-galactic-latitude dark cloud Lynds 1642. Besides thespectrum for the core with $A_V \ge 15$ mag, further spectra were obtained forintermediate-opacity cloud positions. They are used as proxy for the spectrumof the impinging starlight spectrum and facilitate the separation of thescattered starlight (cf. Paper II, Mattila et al. 2017b). Our spectra reach aprecision of $\sim 0.5$ $10^{-9}$ erg cm$^{-2}$s$^{-1}$sr$^{-1}$\AA$^{-1}$ asrequired to measure an EBL intensity in range of $\sim$1 to a few times$10^{-9}$ erg cm$^{-2}$s$^{-1}$sr$^{-1}$\AA$^{-1}$. Because all surfacebrightness components are measured using the same equipment the method does notrequire unusually high absolute calibration accuracy, a condition which hasbeen a problem for some previous EBL projects

Extragalactic Background Light: a measurement at 400 nm using dark cloud shadow I. Low surface brightness spectrophotometry in the area of Lynds 1642