Preheat the oven to 350°F.
Grease an 8x8-inch square pan or a 9-inch round pan.
In a medium bowl, whisk together the flour, baking powder, salt, and cinnamon. Set aside.
In a separate large bowl, whisk together the melted butter, sugar, eggs, and vanilla extract until smooth.
Pour the dry ingredients into the wet ingredients and stir gently until just combined.
Fold in the diced peaches until evenly distributed throughout the batter.
Pour the batter into the prepared pan and spread it evenly.
Bake for 35 to 40 minutes, or until the top is golden and a toothpick inserted in the center comes out clean.
Remove from the oven and let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.