Heat oven to 350°F. Grease and line a 9x13-inch pan.
Toss peaches with lemon juice, granulated sugar, and cornstarch. Set aside to juice.
Stir streusel flour, brown sugar, granulated sugar, cinnamon, and salt in a bowl.
Pour in melted butter. Mix until clumpy; chill while you make the batter.
Whisk batter flour, baking powder, salt, and cinnamon in another bowl.
Cream softened butter and sugar until fluffy, about 2 minutes.
Beat in eggs gradually, then vanilla. Scrape the bowl.
Mix in sour cream until smooth.
Add dry ingredients in two additions, alternating with milk. Mix just until combined.
Fold in peaches with their juices.
Spread batter in pan. Crumble streusel evenly over the top.
Bake 42–48 minutes, until a tester comes out clean. Cool 20 minutes.
Whisk powdered sugar, milk, and vanilla. Drizzle glaze over warm cake. Slice and serve.