Preheat the oven to 350°F. Grease and flour a 9-inch round cake pan.

In a large mixing bowl, combine the cake mix, flour, baking powder, softened butter, and eggs. Beat with an electric mixer on low speed until moistened, then increase speed to medium and beat for two more minutes. Add vanilla extract and mix until well combined.

Pour the batter into the prepared cake pan. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a medium bowl, combine sliced strawberries and granulated sugar. Stir to coat the strawberries evenly. Let them sit for at least 30 minutes to macerate and release their juices.

In a chilled mixing bowl, combine heavy cream and powdered sugar. Beat with an electric mixer on high speed until soft peaks form, about 3-4 minutes.

Slice the cooled cake horizontally into two layers. Place the bottom layer on a serving platter. Spread half of the whipped cream over the cake layer and top with half of the macerated strawberries. Place the second cake layer on top and repeat the process with the remaining whipped cream and strawberries.

Slice the assembled strawberry shortcake into wedges and serve immediately. Enjoy your delicious homemade strawberry shortcake!

Cover any leftover cake and store in the refrigerator for up to 3 days. For best results, store the cake and strawberries separately to prevent the cake from becoming soggy.