Preheat the oven to 350F. Line the bottom of a 9" springform pan with parchment paper.
In a medium bowl, whisk together the flour, baking powder, cinnamon and salt.
In the bowl of a stand mixer fitted with the paddle attachment, on medium speed, beat together the butter and brown sugar, stopping to scrape the bowl every so often, until the mixture is light, fluffy and creamy, about 3-5 minutes.
Beat in the egg and vanilla.
Add the flour mixture to the batter in three batches, alternating with the sour cream, beginning and ending with the flour.
Remove the bowl from the mixer, and scrape down the the bowl and give it a stir to ensure the flour is fully incorporated.
Spoon the batter into the prepared spring form pan and smooth with an offset spatula.
Layer on the nectarines, slightly overlapping, onto the cake batter. Set aside.
In the bowl of a food processor, pulse together the flour, brown sugar, salt, cinnamon and cardamom to combine.
Add the butter and pulse until mixture begins to stick together and form small clumps.
Scatter the streusel onto the cake, evenly covering the nectarines.
Bake the cake for 45-55 minutes, or until a toothpick comes out clean.
Cool the cake in the pan on a wire cooling rack for 15 minutes before removing from the pan. Let cool completely.
In a small bowl, whisk together the powdered sugar, milk and vanilla. Drizzle over the cake. Let the icing firm up for 10 minutes before serving.