Organic solvent nanofiltration (OSN), also known as solvent resistant nanofiltration (SRNF), has enormous potential for industrial applications. Among the emerging high-performance membrane materials, some have proven challenging to fabricate ultrathin selective layers and at a scale large enough to be practical for industrial consideration. This work demonstrates a facile and scalable solution casting method to prepare ultrathin composite films from advanced polymeric materials and porous additives. Using this method, poly(trimethylsilylpropyne) (PTMSP) membranes could be controlled to just 250 nm thickness resulting in 2 to 30-fold higher solvent permeance than other PTMSP membranes reported to date. Two highly porous additives, poly-dichloroxylene (p-DCX) and Porous Aromatic Framework (PAF-1) were employed to fabricate ultrathin PTMSP composite membranes. Compared to the native PTMSP membrane (methanol permeance of 11.5 L m^-2 h^-1 bar^-1), these additives enhanced membrane permeance by 75% (20.1 L m^-2 h^-1 bar^-1) and 50% (17.2 L m^-2 h^-1 bar^-1), respectively. Inclusion of the porous additives also enhanced membrane stability, demonstrated by 300 h of continuous OSN operation. These membranes showed improved molecular weight cut-off (MWCO) and significant potential in the separation of active pharmaceutical ingredients (APIs), including the almost complete rejection of Vitamin B12 in methanol. This work demonstrates that the facile production of ultrathin membranes using intrinsically microporous polymers is possible, enabling these exciting membrane materials to be considered for industrial applications.