2Grease an 8x8 inch square pan and line with parchment paper, allowing paper to extend over the edges.
3In a small bowl, whisk together the flour, baking powder, and salt.
4In a large bowl, whisk together the melted butter and sugar. Once well-incorporated, whisk in the eggs one at a time, followed by the egg yolk, vanilla extract, and buttermilk.
5Gently fold the dry ingredients into the liquid mixture using a spatula until just combined.
6Transfer the batter to the prepared pan and scatter raspberries evenly over the top.
7Sprinkle the cake generously with turbinado sugar.
8Bake for 45 to 50 minutes or until a toothpick inserted in the middle comes out clean.
9Let the cake cool in the pan on a wire rack for 10 minutes, then lift out using the parchment paper and let cool completely.