Preheat and prepare pan: Preheat oven to 350°F (175°C). Lightly grease a 9-inch round cake pan or 9x2-inch round pan. Pour melted butter into the pan and spread evenly. Sprinkle brown sugar over the butter.

Arrange fruit: Tuck pineapple chunks into the sugar-butter layer. Scatter blueberries around and in between the pineapple pieces.

Make batter: In a large bowl, whisk together flour, baking powder, and salt. In another bowl, cream the softened butter with granulated sugar until fluffy. Beat in eggs one at a time, then mix in vanilla. Alternately add dry ingredients and milk in 2–3 additions, starting and ending with dry ingredients, until just combined.

Combine and bake: Gently fold in remaining blueberries (to avoid a purple-mushroom effect, fold carefully). Pour batter over the fruit in the pan, spreading evenly.

Bake: Bake 40–50 minutes, or until a toothpick inserted into the center comes out clean. If the top gets too brown, cover loosely with foil for the last 10 minutes.

Cool and invert: Let the cake cool in the pan on a rack for about 10 minutes. Run a knife around the edge, then invert onto a serving plate. Let it cool slightly before slicing.