Preheat oven to 350°F (175°C). Grease a 13×18-inch baking sheet pan.
In a mixer or large bowl, beat together 1 cup sugar, 1 cup plain Greek yogurt, and 1/2 cup softened butter until light and fluffy.
Add 2 large eggs, 2 cups mashed bananas (about 4 medium bananas), and 2 teaspoons vanilla. Mix until well combined and smooth.
Add 2 cups flour, 1 teaspoon baking soda, and 1 teaspoon salt. Mix just until combined and no streaks of flour remain.
Pour the batter into the prepared sheet pan and spread it evenly.
Bake 25–30 minutes, until the top is golden brown and springs back when lightly pressed.
Remove the cake from the oven and let it cool in the pan for about 10 minutes (the cake should be warm but not hot when you frost it).
While the cake cools, brown 1/4 cup butter for the frosting: cut the 1/4 cup butter into pieces and cook in a small sauté pan over medium heat, stirring constantly for about 5–8 minutes, until it foams and turns golden brown. Remove from heat and pour the browned butter into a heatproof bowl to stop the cooking; let it cool until warm (not hot).
Add 1 cup confectioners’ sugar, 2 tablespoons milk, and 1 teaspoon vanilla to the warm browned butter. Beat until the frosting is smooth and creamy.
Pour the frosting over the warm (not hot) cake and spread to cover the surface evenly.
Slice the cake into 24 even pieces. Serve warm or cool.