Preheat oven to 350°F (175°C). Grease a 9-inch baking pan.
In a bowl, combine brown sugar, flour, and cinnamon for streusel. Cut in cold butter until crumbly. Set aside.
In a medium bowl, whisk flour, baking powder, baking soda, and salt.
In a large bowl, cream butter and sugar until fluffy.
Add eggs and vanilla extract. Mix well.
Stir in sour cream.
Gradually add dry ingredients and mix until combined.
Fold in diced peaches gently.
Spread batter into prepared pan.
Sprinkle streusel topping evenly over batter.
Bake 40–50 minutes until golden and toothpick comes out mostly clean.
Cool slightly before slicing.
If desired, whisk glaze ingredients together and drizzle over cake.