diff --git a/aws_lambda_powertools/logging/formatter.py b/aws_lambda_powertools/logging/formatter.py
index cb3bb397348..bb0d9865f2c 100644
--- a/aws_lambda_powertools/logging/formatter.py
+++ b/aws_lambda_powertools/logging/formatter.py
@@ -3,13 +3,13 @@
 from typing import Any
 
 
-def json_formatter(unserialized_value: Any):
-    """JSON custom serializer to cast unserialisable values to strings.
+def json_formatter(unserializable_value: Any):
+    """JSON custom serializer to cast unserializable values to strings.
 
     Example
     -------
 
-    **Serialize unserialisable value to string**
+    **Serialize unserializable value to string**
 
         class X: pass
         value = {"x": X()}
@@ -18,10 +18,10 @@ class X: pass
 
     Parameters
     ----------
-    unserialized_value: Any
+    unserializable_value: Any
         Python object unserializable by JSON
     """
-    return str(unserialized_value)
+    return str(unserializable_value)
 
 
 class JsonFormatter(logging.Formatter):
@@ -39,11 +39,12 @@ def __init__(self, **kwargs):
         """Return a JsonFormatter instance.
 
         The `json_default` kwarg is used to specify a formatter for otherwise
-        unserialisable values.  It must not throw.  Defaults to a function that
+        unserializable values.  It must not throw.  Defaults to a function that
         coerces the value to a string.
 
         Other kwargs are used to specify log field format strings.
         """
+        self.default_json_formatter = kwargs.pop("json_default", json_formatter)
         datefmt = kwargs.pop("datefmt", None)
 
         super(JsonFormatter, self).__init__(datefmt=datefmt)
@@ -54,7 +55,6 @@ def __init__(self, **kwargs):
             "location": "%(funcName)s:%(lineno)d",
         }
         self.format_dict.update(kwargs)
-        self.default_json_formatter = kwargs.pop("json_default", json_formatter)
 
     def update_formatter(self, **kwargs):
         self.format_dict.update(kwargs)
@@ -64,6 +64,7 @@ def format(self, record):  # noqa: A003
         record_dict["asctime"] = self.formatTime(record, self.datefmt)
 
         log_dict = {}
+
         for key, value in self.format_dict.items():
             if value and key in self.reserved_keys:
                 # converts default logging expr to its record value
@@ -84,19 +85,13 @@ def format(self, record):  # noqa: A003
             except (json.decoder.JSONDecodeError, TypeError, ValueError):
                 pass
 
-        if record.exc_info:
+        if record.exc_info and not record.exc_text:
             # Cache the traceback text to avoid converting it multiple times
             # (it's constant anyway)
             # from logging.Formatter:format
-            if not record.exc_text:  # pragma: no cover
-                record.exc_text = self.formatException(record.exc_info)
+            record.exc_text = self.formatException(record.exc_info)
 
         if record.exc_text:
             log_dict["exception"] = record.exc_text
 
-        json_record = json.dumps(log_dict, default=self.default_json_formatter)
-
-        if hasattr(json_record, "decode"):  # pragma: no cover
-            json_record = json_record.decode("utf-8")
-
-        return json_record
+        return json.dumps(log_dict, default=self.default_json_formatter)
diff --git a/tests/functional/test_aws_lambda_logging.py b/tests/functional/test_aws_lambda_logging.py
index cf4782d1d2a..86c6124cb1f 100644
--- a/tests/functional/test_aws_lambda_logging.py
+++ b/tests/functional/test_aws_lambda_logging.py
@@ -38,16 +38,17 @@ def test_setup_with_valid_log_levels(stdout, level):
 
 
 def test_logging_exception_traceback(stdout):
-    logger = Logger(level="DEBUG", stream=stdout, request_id="request id!", another="value")
+    logger = Logger(level="DEBUG", stream=stdout)
 
     try:
-        raise Exception("Boom")
-    except Exception:
-        logger.exception("This is a test")
+        raise ValueError("Boom")
+    except ValueError:
+        logger.exception("A value error occurred")
 
     log_dict = json.loads(stdout.getvalue())
 
     check_log_dict(log_dict)
+    assert "ERROR" == log_dict["level"]
     assert "exception" in log_dict
 
 
@@ -86,15 +87,32 @@ def test_with_json_message(stdout):
     assert msg == log_dict["message"]
 
 
-def test_with_unserialisable_value_in_message(stdout):
+def test_with_unserializable_value_in_message(stdout):
     logger = Logger(level="DEBUG", stream=stdout)
 
-    class X:
+    class Unserializable:
         pass
 
-    msg = {"x": X()}
+    msg = {"x": Unserializable()}
     logger.debug(msg)
 
     log_dict = json.loads(stdout.getvalue())
 
     assert log_dict["message"]["x"].startswith("<")
+
+
+def test_with_unserializable_value_in_message_custom(stdout):
+    class Unserializable:
+        pass
+
+    # GIVEN a custom json_default
+    logger = Logger(level="DEBUG", stream=stdout, json_default=lambda o: f"<non-serializable: {type(o).__name__}>")
+
+    # WHEN we log a message
+    logger.debug({"x": Unserializable()})
+
+    log_dict = json.loads(stdout.getvalue())
+
+    # THEN json_default should not be in the log message and the custom unserializable handler should be used
+    assert log_dict["message"]["x"] == "<non-serializable: Unserializable>"
+    assert "json_default" not in log_dict