Heat oven to 350°F. Grease and line an 8-inch square or 9-inch round pan with parchment.
Toss the diced peaches with lemon juice and set aside to let the juices settle.
Whisk flour, baking powder, baking soda, salt, and cinnamon in a medium bowl.
Cream butter, brown sugar, and granulated sugar in a large bowl until fluffy, 2–3 minutes.
Beat in the eggs a little at a time, then mix in vanilla until smooth.
Blend in sour cream. Scrape the bowl to keep the batter even.
Add dry ingredients in two additions, alternating with the milk. Mix just until combined.
Fold in the peaches and any juices gently to avoid overmixing.
Spread batter into the pan and smooth the top. Sprinkle with turbinado sugar.
Bake 38–44 minutes, until a toothpick comes out clean or with a few moist crumbs.
Cool 15 minutes in the pan, then turn out or slice and serve warm.