Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper leaving a 2-inch overhang at each side.
For the crumb topping, combine flour, both sugars, and salt in a medium bowl. Add melted butter and stir until evenly combined.
For the cake batter, whisk melted butter, both sugars, and vanilla until pale and fluffy. Mix in eggs, then yogurt. Sift flour, baking powder, and salt, then fold into the wet mixture. Add milk and fold until combined.
Spread batter into the prepared pan. Toss chopped peaches with flour and sugar, then scatter evenly over the batter. Top with crumb topping.
Bake for 40-50 minutes until lightly golden. Cool on a wire rack before serving.