Preheat your oven to 350°F (175°C). Grease and lightly flour an 8×8 or 9×9-inch square baking pan, or line it with parchment paper.
In a small bowl, combine the flour, brown sugar, and cinnamon. Add in the cold butter cubes and work the butter into the dry ingredients until the mixture resembles coarse crumbs.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large mixing bowl, beat the softened butter with the granulated sugar and brown sugar until light and fluffy—about 2–3 minutes with a mixer.
Beat in each egg one at a time, then add vanilla extract and sour cream (or Greek yogurt). Mix until smooth.
Gradually mix in your previously whisked dry ingredients into your wet mixture. Stir just until no flour streaks remain.
Gently fold in your diced peaches using a spatula.
Spread your batter evenly into your prepared pan and sprinkle on the cold cinnamon streusel topping.
Bake for about 40–50 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs attached.
Let your cake cool in its pan for at least 15–20 minutes before slicing. If adding a glaze, whisk together powdered sugar, vanilla extract, and milk until smooth; then drizzle over the cooled cake.