Preheat oven to 400F. Lightly spray a 9 × 9-in. baking dish with oil. Set aside.
Stir flour with baking powder and salt in a medium bowl. Beat 2 tbsp butter with 1/2 cup sugar and vanilla in a large bowl with a wooden spoon. Add one-third of flour mixture to butter mixture, then add half the milk. Repeat additions, scraping down sides of bowl, until well mixed. Gently stir in berries. Scrape batter into prepared pan and smooth top.
Bake in centre of oven until cake is lightly golden, about 30 min. Poke cake a few times with a fork and set aside.
To make sauce: Combine butter with sugar, cream and whisky in a small saucepan and set over medium-high. Bring to a boil, then reduce heat to medium-low. Simmer, stirring occasionally, until smooth, about 5 min. Drizzle sauce over warm cake. Let stand for 10 min. Serve warm, topped with whipped cream.