Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or nonstick spray and set aside.
In a large bowl, toss the sliced peaches with 1/4 cup granulated sugar and the cinnamon. Let sit for 15 minutes to macerate and release juices.
In a medium bowl, whisk together the flour, remaining 1/2 cup granulated sugar, baking powder, and salt until evenly combined.
In a separate large bowl, whisk together the melted butter, milk, eggs, and vanilla extract until smooth and well blended.
Add the dry ingredients to the wet ingredients and stir gently with a spatula just until no flour streaks remain. Do not overmix.
Pour the batter into the prepared baking dish and spread it evenly with a spatula. Arrange the peach slices and their juices over the top of the batter in an even layer.
Sprinkle the brown sugar evenly over the peaches, then finish with the turbinado sugar for a crunchy topping.
Bake for 50-60 minutes, until the cake is golden brown and a toothpick inserted into the cake portion (not the fruit) comes out clean. The edges should be bubbling and caramelized.
Remove from the oven and let cool for 15-20 minutes before serving. The cake will continue to set as it cools.
Serve warm with vanilla ice cream or whipped cream. Store any leftovers covered at room temperature for up to 2 days or refrigerate for up to 5 days.
