  Josephson tunneling in twisted cuprate junctions provides a litmus test for
the pairing symmetry, which is fundamental for understanding the microscopic
mechanism of high temperature superconductivity. This issue is rekindled by
experimental advances in van der Waals stacking and the proposal of an emergent
d+id-wave. So far, all experiments have been carried out on
Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi-2212) with double CuO$_2$ planes but show
controversial results. Here, we investigate junctions made of
Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+y}$ (Bi-2201) with single CuO$_2$ planes. Our
on-site cold stacking technique ensures uncompromised crystalline quality and
stoichiometry at the interface. Junctions with carefully calibrated twist
angles around 45{\deg} show strong Josephson tunneling and conventional
temperature dependence. Furthermore, we observe standard Fraunhofer diffraction
patterns and integer Fiske steps in a junction with a twist angle of
45.0$\pm$0.2{\deg}. Together, these results pose strong constraints on the d or
d+id-wave pairing and suggest an indispensable isotropic pairing component.
