Combine butter, sugar, half & half, and almond extract in a medium saucepan.
Heat over medium heat, stirring, until mixture comes to a full boil. Boil 1 minute, stirring constantly, then remove from heat.
Let stand 2-3 minutes to cool slightly. Gradually add powdered sugar until mixture is thickened but still pourable.
Pour over muffins to glaze; top with additional sliced almonds (optional).