mac – unable to install bitcoind (macOS)

I’ve been trying to install bitcoind on a new mac (Big Sur 11.4). I followed the instructions in doc/build-osx.md and installed all required dependencies, but when I try to run make, it gives me the following error:

Making all in src
  CXX      bitcoind-bitcoind.o
clang: warning: argument unused during compilation: '-fstack-clash-protection' (-Wunused-command-line-argument)
In file included from bitcoind.cpp:14:
In file included from ./interfaces/chain.h:8:
./optional.h:10:10: fatal error: 'boost/optional.hpp' file not found
#include <boost/optional.hpp>
         ^~~~~~~~~~~~~~~~~~~~
1 error generated.
make(2): *** (bitcoind-bitcoind.o) Error 1
make(1): *** (all-recursive) Error 1
make: *** (all-recursive) Error 1

Any idea how to fix this?

Edit:
If I run brew install boost, it tells me it’s already installed, but typing ‘which boost’ gives ‘boost not found’.