Mary Berg's Brown Sugar Slice & Bake Cookies

Ingredients

• 1 1/4 cup unsalted butter, room temperature
• 3/4 cup dark brown sugar
• 1/4 cup sugar
• 2 teaspoons vanilla extract
• 1 egg white
• 2 3/4 cups all-purpose flour
• 1 teaspoon kosher salt
• 1/4 teaspoon baking soda
• Jam, buttercream, melted chocolate, or colourful sugar, for decorating