From Hershey's
Unlike a fudgy cake, which relies on whipped egg foam for lift, dense chocolate squares demand minimal incorporation to avoid toughness. Overbeating the flour after it joins the wet ingredients develops gluten, turning a tender crumb into a chewy, resilient bread. Melt the butter completely before introducing the sugar, as this dissolved state encourages a shiny, crackled top crust during baking. Add the eggs individually, beating thoroughly after each addition to emulsify the fats and liquids into a homogeneous base. Watch the edges carefully in a 350-degree oven; baking for 30 to 35 minutes works, but pulling the pan the moment the sides shrink inward prevents the center from drying out. Cool the slab entirely on a wire rack before slicing with a long, thin blade wiped clean between cuts. Stored in an airtight container at room temperature, these squares hold their texture for up to three days, or freeze them individually wrapped in plastic for up to two months without losing structural integrity.