Preheat your oven to 350°F (175°C). Grease and line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.
Add the softened butter, egg, vanilla extract, and milk to the dry ingredients. Mix until just combined and the batter is smooth.
Pour the batter into the prepared baking pan and spread it evenly. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool completely in the pan.
While the cake is cooling, combine the sliced strawberries and lemon juice in a bowl. Let them sit for about 10-15 minutes to macerate.
In a separate bowl, whip the heavy cream, powdered sugar, and vanilla extract until soft peaks form.
Once the cake has cooled, spread the whipped cream over the top evenly. Then, layer the macerated strawberries on top of the whipped cream.
Use the parchment paper to lift the cake out of the pan. Cut into bars and serve immediately or refrigerate until ready to serve.