Etwas fragmentarisch, damit's übersichtlich bleibt (sozusagen zum »Selber ausmalen«).

<div class="beispiel-css-menue">
	<ul class="ul-menu">
		<li class="close">
			<label class="label" for="close"> X <input id="close" name="mainmenu" type="radio"></label>
		</li>
		<li class="li-menu">	
			<label class="label" for="kuchen"><b>❦</b> Kuchen <input id="kuchen" name="mainmenu" type="radio">
			<ul class="sub">
				<li>festliche Torten</li>
				<li>Schokoladensahne</li>
				<li>Nusskuchen Emma</li>
				<li>Eissplitter Natur</li>
			</ul>
			</label>
		</li>
		<li class="li-menu">
			<label class="label" for="gebäck"><b>❦</b> Gebäck <input id="gebäck" name="mainmenu" type="radio"> 
			<ul class="sub">
				<li>Sonntagsbrötchen</li>
				<li>Nussplätzchen</li>
				<li>Feinster Lebkuchen</li>
				<li>Mandelbiscuit</li>
				<li>Caramel Muffin</li>
			</ul>
			</label>
		</li>
		<li class="li-menu">
			<label class="label" for="brot"><b>❦</b> Brote <input id="brot" name="mainmenu" type="radio"> 
			<ul class="sub">
				<li>Tiroler Steinofen</li>
				<li>Paderborner</li>
				<li>Feiertagsstuten</li>
			</ul>
			</label>
		</li>
	</ul>
</div>

<style>
.beispiel-css-menue {
	position: relative;
	padding-top: 10px;
	padding-right: 10px;
}

.beispiel-css-menue ul {
	list-style-type: none;
	padding-left: 1rem;
}

.beispiel-css-menue ul li:not(:last-child) {
	border-bottom: 1px dotted silver;
}

.beispiel-css-menue li.close {
	position: absolute;
	color: red;
	font-weight: bold;
	right: 20px;
	top: 40px;
	z-index: 1000;
}

.li-menu {
	background: #333;
	padding: 1rem;
	border-bottom: 1px dotted #fff;
	color: #fff;
}

.sub {
	max-height: 0;
	overflow: hidden;
}

.sub li {
	padding: 1rem 0.5rem;
}

input[name="mainmenu"] {
	display: none;
}

input[name="mainmenu"]:checked + .sub {
	max-height: 1000px;
	overflow: visible;
}

@media (min-width: 720px) {
	.ul-menu {
		display: flex;
	}
	
	.li-menu {
		position: relative;
		width: 30%;
	}
	
	.sub {
		position: absolute;
		background: #444;
		padding-right: 2rem;
		padding-left: 1rem;
	}
}
</style>