Hey @trickykid, Bash and (Windows) PowerShell are similar in ways but not identical, thus you should use different syntax (?)/functions. The following espanso command works for me on Windows 10:
# Breadcrumb
- trigger: ":bc"
word: true
replace: "{{output}}"
vars:
- name: output
type: shell
params:
cmd: (echo "[[$((Get-Date).AddDays(-1) | Get-Date -UFormat "%Y-%m-%d (%a)")|<< Yesterday]] | [[$((Get-Date).AddDays(+1) | Get-Date -UFormat "%Y-%m-%d (%a)")|Tomorrow >>]]")