3 sticks (3/4 lb.) unsalted butter, at room temperature
1 pound light brown sugar
5 large eggs, separated, at room temperature
1/2 cup sugar
1 cup confectioners' sugar
2 tablespoons milk
1/4 teaspoon vanilla extract
How to Make It
Preheat oven to 350°F. Butter and flour a 10-inch tube pan or Bundt pan.
Make cake: Mix dry ingredients in a bowl. Whisk buttermilk and vanilla in a small bowl. Using an electric mixer on medium-high speed, beat butter and brown sugar until fluffy. Beat in egg yolks, 1 at a time. Reduce speed to low; alternately add flour mixture and buttermilk, starting and ending with flour.
In a dry bowl, beat egg whites on medium-high speed until soft peaks form. Add sugar 1 Tbsp. at a time, beating until whites form stiff peaks. Fold 1/3 of egg whites into batter, then fold in remaining whites. Transfer batter to pan.
Bake 75 to 85 minutes, until a toothpick inserted in center comes out clean. Let cake stand in pan 10 minutes, invert onto wire rack; let cool.
Make glaze: Mix sugar, milk and vanilla in a bowl until smooth. Spoon glaze atop cake.