Fresh Apple Crumb Cake, first inspired by Smitten Kitchen

Heat oven & Prepare pan(x): To 325°F (165°C). Lightly coat an 8-inch square or 9-inch cake pan with butter. Line pan with parchment paper, coating the top of the parchment as well.
Prepare apples by tossing with lemon juice and then sugar and cinnamon. Set aside at room temperature while preparing the rest.
Make topping: Blend butter, sugars, cinnamon, and salt together until evenly mixed. Add flour and mix until it disappears. It’s going to be very thick; set it it aside.
Make cake: Beat second butter with sugar until lightened and fluffy. Add egg, sour cream, and vanilla and beat until well combined. Sprinkle surface of batter with with baking powder and salt, and mix well to combine. Add flour and mix only until it disappears.
Assemble: Scrape batter into prepared cake pan and smooth it flat. Arrange apples on cake, slightly overlapped. I usually fit all but 2 wedges; those are cook’s snacks. Pour any cinnamon-apple juices in bottom of bowl over apples. Sprinkle crumbs over apple slices. For bigger crumbs, squeeze the crumbs into small fistfuls and break these up into a couple bigger chunks over the cake.
Bake: Bake the cake until a apples are tender and bubbling around edge of pan, about 50 to 55 minutes.
Cool to room temperature before cutting. Store at room temperature in air tight container, or in the refrigerator well wrapped.