Preheat oven to 350 degrees and put cupcake papers into two cupcake tins (this recipe makes 18 cupcakes).
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
In the bowl of a stand mixer or in a large bowl with a hand mixer, cream together butter and sugar on medium-high speed until light and fluffy, about 3 minutes.
Add eggs in one at a time, mixing well after each addition. Mix in vanilla extract.
With mixer on low speed, beat in dry ingredients in 3 additions, alternating with Greek yogurt/sour cream and milk. Mix until just incorporated, taking care not to over-mix. Fold in diced peaches.
Fill cupcake liners about ¾ of the way with batter and bake for 17-20 minutes, until edges are golden and tester comes out clean. Let cupcakes cool in pan for a few minutes before removing to wire racks to cool completely.
Once cupcakes are completely cooled, core out a small section from the middle (either using a cupcake corer or with a knife). Fill center of cupcake with peach filling. Be sure to reserve about 3 tablespoon of peach filling for frosting.
Frost cupcakes (I used a Wilton 1M pastry tip) and top them with a peach slice and a drizzle of peach filling, if desired.