Preheat your oven to 350°F (175°C) and grease and lightly flour an 8×8 or 9×9-inch square baking pan.
In a small bowl, combine the flour, brown sugar, and cinnamon. Add cold butter cubes and mix until it resembles coarse crumbs. Refrigerate the topping.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large mixing bowl, beat together softened butter, granulated sugar, and brown sugar until light and fluffy.
Beat in each egg one at a time, then add vanilla extract and sour cream, mixing until smooth.
Gradually mix the dry ingredients into the wet mixture until no flour streaks remain.
Gently fold in the diced peaches into the batter.
Spread the batter evenly into the prepared pan and sprinkle the streusel topping over it.
Bake for about 40–50 minutes, checking for doneness with a toothpick.
Let the cake cool in the pan for at least 15–20 minutes before slicing. Optionally, whisk together powdered sugar, vanilla extract, and milk for a glaze and drizzle over the cooled cake.