Preheat oven to 350°F (180°C). Grease a 9-inch round cake pan and line the base with parchment paper.
In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
In a large bowl, beat softened butter for 1 minute until creamy. Add sugar and beat for 1 more minute until fluffy and pale.
Add eggs one at a time, beating well after each addition. Mix in vanilla extract.
Add flour mixture and beat on low just until no dry flour remains. Do not overmix.
Fold in diced apples with a spatula until evenly combined. Batter will be very thick — this is normal.
Spread batter into prepared pan and smooth the top. Bake for 35–45 minutes until a toothpick in the center comes out clean.
Rest in pan for 5 minutes, then turn out onto a wire rack. Cool 10 more minutes. Dust with powdered sugar and serve warm or at room temperature.