diff --git a/src/test/java/org/kiwiproject/test/okhttp3/mockwebserver/MockWebServerExtension.java b/src/test/java/org/kiwiproject/test/okhttp3/mockwebserver/MockWebServerExtension.java
index 7b4da39..fcfa6db 100644
--- a/src/test/java/org/kiwiproject/test/okhttp3/mockwebserver/MockWebServerExtension.java
+++ b/src/test/java/org/kiwiproject/test/okhttp3/mockwebserver/MockWebServerExtension.java
@@ -1,5 +1,9 @@
 package org.kiwiproject.test.okhttp3.mockwebserver;
 
+import static java.util.Objects.isNull;
+import static org.kiwiproject.base.KiwiPreconditions.requireNotNull;
+
+import lombok.Builder;
 import lombok.Getter;
 import lombok.experimental.Accessors;
 import okhttp3.mockwebserver.MockWebServer;
@@ -7,8 +11,10 @@
 import org.junit.jupiter.api.extension.BeforeEachCallback;
 import org.junit.jupiter.api.extension.ExtensionContext;
 import org.kiwiproject.io.KiwiIO;
+import org.kiwiproject.util.function.KiwiConsumers;
 
 import java.io.IOException;
+import java.util.function.Consumer;
 
 /**
  * A simple JUnit Jupiter extension that creates and starts a {@link MockWebServer}
@@ -18,11 +24,38 @@ public class MockWebServerExtension implements BeforeEachCallback, AfterEachCall
 
     @Getter
     @Accessors(fluent = true)
-    private MockWebServer server;
+    private final MockWebServer server;
+
+    private final Consumer<MockWebServer> serverCustomizer;
+
+    /**
+     * Create a new instance.
+     * <p>
+     * The extension will use a default {@link MockWebServer} instance.
+     * <p>
+     * If you want to customize the server or provide your own {@link MockWebServer},
+     * use the builder instead of this constructor.
+     */
+    public MockWebServerExtension() {
+        this(new MockWebServer(), KiwiConsumers.noOp());
+    }
+
+    /**
+     * Create a new instance that will use the given {@link MockWebServer} and customizer.
+     *
+     * @param server           the server
+     * @param serverCustomizer allows a test to configure the server, e.g., to customize the protocols
+     *                         it supports or to serve requests via HTTPS over TLS.
+     */
+    @Builder
+    MockWebServerExtension(MockWebServer server, Consumer<MockWebServer> serverCustomizer) {
+        this.server = requireNotNull(server, "server must not be null");
+        this.serverCustomizer = isNull(serverCustomizer) ? KiwiConsumers.noOp() : serverCustomizer;
+    }
 
     @Override
     public void beforeEach(ExtensionContext context) throws IOException {
-        server = new MockWebServer();
+        serverCustomizer.accept(server);
         server.start();
     }