In a large bowl, combine strawberries and 3 tablespoons sugar and toss until coated. Refrigerate, 30 minutes, or up to 1 hour.
Preheat oven to 400° and spray an 8”-x-8” baking pan with cooking spray. Stir together flour, remaining 1/4 cup sugar, baking powder, baking soda, and salt in a large bowl. Pour in heavy cream and stir until just combined.
Drop dough into prepared pan and press down evenly. Bake until edges are golden and a toothpick comes out clean, 18 to 20 minutes. Let cool.
Meanwhile, make whipped cream topping: In a large bowl using a hand mixer or in the bowl of a stand mixer using the whisk attachment, beat heavy cream and vanilla until soft peaks form, 2 to 3 minutes. Keep chilled until ready to serve.
To serve, slice cake into 12 pieces. Top bottom shortcake with sugared strawberries, then a second shortcake. Dollop with whipped cream.