Soft Butter Cake

Ingredients
Cooking spray
4 large eggs, room temperature
1 cup (200 grams) granulated sugar, divided
2 sticks (16 tablespoons/8 ounces/227 grams) good-quality unsalted butter
1 teaspoon vanilla extract
1/4 to 1/2 teaspoons finely grated lime, lemon, or orange zest (optional)
1 1/2 cups (200 grams) all-purpose flour (my favorite is Pillsbury bleached all-purpose flour; do not use cake flour), divided
2 1/4 teaspoons baking powder
1/4 cup (60 milliliters) whole milk, room temperature, divided
Directions
Heat the oven to 335°F. Spray an 8-inch metal square, preferably light-colored, pan with cooking spray.
Separate the egg whites from the yolks. In a large bowl, using an electric mixer on medium-high speed, beat the egg whites until soft peaks form. Gradually add ¼ cup (50 grams) of the sugar and beat until stiff. Set aside (transfer to another bowl if you need the same bowl to cream the butter).
In another large bowl, using the electric mixer on medium-high speed, cream the butter and the remaining ¾ cup (150 grams) of the sugar until and fluffy. Be patient; it may take 5 to 10 minutes. Add the vanilla and lime zest, if using. Add the eggs yolks one at a time, gradually beating for a few seconds to incorporate after each addition.
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