index.html

<label for="story">Put your base64 here</label>

<script>
	const b64toBlob = (b64Data, contentType = '', sliceSize = 512) => {
		const byteCharacters = atob(b64Data);
		const byteArrays = [];

		for (
			let offset = 0;
			offset < byteCharacters.length;
			offset += sliceSize
		) {
			const slice = byteCharacters.slice(offset, offset + sliceSize);

			const byteNumbers = new Array(slice.length);
			for (let i = 0; i < slice.length; i++) {
				byteNumbers[i] = slice.charCodeAt(i);
			}

			const byteArray = new Uint8Array(byteNumbers);
			byteArrays.push(byteArray);
		}

		const blob = new Blob(byteArrays, { type: contentType });
		return blob;
	};

	function download() {
		const b64Data = document.getElementById('source').value;
		const contentType = 'application/octet-stream';
		const blob = b64toBlob(b64Data, contentType);
		const blobUrl = URL.createObjectURL(blob);

		window.location = blobUrl;
	}
</script>

<textarea id="source"></textarea>
<button onclick="download()">Download</button>