Apple Cider Muffins with Cinnamon Streusel

Reduce the apple cider: Pour 2 cups into a saucepan. Simmer over medium heat until reduced to ½ cup (about 30-40 minutes). Let cool for at least 10 minutes.
Preheat oven to 350°F (177°C). Spray a 12-count muffin pan with nonstick spray or line with cupcake liners. You may need a 2nd pan as this recipe makes up to 14 muffins, though you can always bake in batches using 1 pan. Set aside.
Make the streusel topping: Combine flour, brown sugar, and cinnamon in a bowl. Add the cold butter pieces and mix in until crumbly. I found that using my hands for this is the easiest way. It does take several minutes to come together. Set aside.
Muffin mix: In a medium bowl, whisk together flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
Mix wet ingredients: In a large bowl, whisk melted butter with both sugars until combined. Whisk in eggs one at a time until combined.
Whisk in cooled cider reduction, sour cream, and vanilla until combined.
Whisk dry ingredients into wet ingredients until just combined. Add diced apple and fold it into the batter.
Fill the muffin cups to the top. Sprinkle with streusel topping
Bake for 18-22 minutes or until a toothpick comes out clean. Cool in pan for 5 minutes, then transfer to wire rack
To make the icing: In a small bowl, whisk together confectioners' sugar, heavy cream, and vanilla extract until smooth. Drizzle over cooled muffins. Let the icing set for about 10 minutes before serving.