1) Preheat oven to 350 degrees and lightly butter an 8×8 baking dish.
2) Stir together streusel ingredients. Mash butter into the streusel to make sure it is evenly distributed. Set aside until needed.
3) In a stand mixer with the paddle attachment or with a hand mixer, cream together butter and sugar for the batter. Meanwhile whisk together dry ingredients separately (flour, baking powder, baking soda, salt).
4) Once butter/sugar are light and fluffy, add in eggs, one at a time, and then mix in sour cream, vanilla, and cinnamon.
5) Stir in dry ingredients in two batches and mix until just combined. Try not to overmix the batter.
6) Pour half the batter into your buttered 8×8 dish. Then top with 2/3 of the streusel mixture. Then top with the other half of batter. Finish the cake with the rest of the streusel and top the cake with sliced peaches.
7) Bake coffee cake at 350 degrees for 55-60 minutes. Make sure the center of the cake is cooked through with a cake tester.
8) Let cake cool for 10-15 minutes and then slice and serve. Can be made a day in advance without a problem.