Preheat oven to 350°F and set aside a greased 10”x15” pan.
In a large bowl, beat the butter and sugar until light and pale yellow.
Add eggs one at a time, mixing well after each addition.
Add all-purpose flour, baking powder, and vanilla. Whisk until smooth.
Pour batter into prepared pan and bake for 35 minutes. Remove cake and allow it to cool for 30 minutes. Reduce oven temperature to 300°F.
Cut the cake down the center horizontally, and then cut into ½” slices. Line the sliced cake on an ungreased baking sheet, sliced side-up, and bake for 30 minutes. Flip the slices over and bake for an additional 30 minutes.
Cool on baking sheets to room temperature.