diff --git a/tests/venv/start/postBuild/README.rst b/tests/venv/start/postBuild/README.rst
new file mode 100644
index 00000000..1a9851f3
--- /dev/null
+++ b/tests/venv/start/postBuild/README.rst
@@ -0,0 +1,8 @@
+System - Post-build scripts
+---------------------------
+
+It is possible to run scripts after you've built the environment specified in
+your other files. This could be used to, for example, download data or run
+some configuration scripts.
+
+In this example, we download and install a Jupyter Notebook extension.
diff --git a/tests/venv/start/postBuild/postBuild b/tests/venv/start/postBuild/postBuild
new file mode 100755
index 00000000..28b840f0
--- /dev/null
+++ b/tests/venv/start/postBuild/postBuild
@@ -0,0 +1,2 @@
+#!/bin/bash
+jupyter nbextension enable --py --sys-prefix ipyleaflet
diff --git a/tests/venv/start/postBuild/requirements.txt b/tests/venv/start/postBuild/requirements.txt
new file mode 100644
index 00000000..5a2f5096
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+++ b/tests/venv/start/postBuild/requirements.txt
@@ -0,0 +1 @@
+ipyleaflet
diff --git a/tests/venv/start/postBuild/start b/tests/venv/start/postBuild/start
new file mode 100644
index 00000000..c458c91a
--- /dev/null
+++ b/tests/venv/start/postBuild/start
@@ -0,0 +1,5 @@
+#!/bin/bash
+
+export TEST_START_VAR="var is set"
+
+exec "$@"
diff --git a/tests/venv/start/postBuild/verify b/tests/venv/start/postBuild/verify
new file mode 100755
index 00000000..cc0d9ea7
--- /dev/null
+++ b/tests/venv/start/postBuild/verify
@@ -0,0 +1,9 @@
+#!/bin/bash
+set -euo pipefail
+jupyter nbextension list | grep 'jupyter-leaflet' | grep enabled
+
+if [ "$TEST_START_VAR" != "var is set" ]
+then
+  echo "TEST_START_VAR is not set"
+  exit 1
+fi