Preheat the oven to 350°F (175°C). Grease and line an 8-inch round or square cake pan with parchment paper to ensure the cake releases easily after baking.

In a small bowl, mix together the brown sugar, flour, cinnamon, and salt. Add the melted butter and stir with a fork until small clumps form. Set aside while you prepare the cake batter.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside for later use.

In a large mixing bowl, beat the softened butter and granulated sugar until light and fluffy, about 2 to 3 minutes, using a hand mixer or stand mixer.

Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract until fully combined.

Mix in the sour cream. Then add the dry ingredients in two additions, mixing just until the batter comes together. Avoid overmixing to keep the cake tender.

Pour the batter into the prepared pan and smooth the top with a spatula. Arrange the peach slices evenly over the surface of the batter. Sprinkle the cinnamon streusel evenly over the peaches.

Bake for 40 to 50 minutes, or until a toothpick inserted into the center comes out clean and the top is golden. The cake should spring back slightly when gently pressed.

Allow the cake to cool in the pan for 10 minutes. Then lift it out using the parchment paper and transfer to a wire rack to cool completely. Dust with powdered sugar before serving, if desired.