To make the cake, slice the lemon and put the slices in a pan of simmering water for 1 minute. Drain and repeat. Arrange the lemon slices in a single layer on a parchment lined baking sheet.
In the meantime, preheat the oven to 350 degrees and butter a 9-inch angel food pan. Or in my case open the liners and put them on a pan. I used 8 but could have stretched it to 9..
Whisk together the flour, baking powder, soda, and salt. Set aside. Cream together the butter, sugar, and lemon zest. Beat until light and fluffy, about 2 minutes. Add the eggs one at a time. Add the vanilla. With the mixer on low add the flour in 3 additions alternating with the sour cream.
Spoon half of the mixture into the cake pan. Arrange half of the lemon slices in a single layer over the batter. Add the renaming batter and cover with the remaining lemon slices. Sprinkle the streusel evenly over the batter.
Bake until the cake is golden brown and a tester comes out clean, about 55 minutes. (The individual pans took about 33 minutes.) Let cool for 15 minutes. Run a knife around the edges of the pan and remove the outer ring. Let cool for 15 minutes. Run a knife around the center ring. Slide 2 wide spatulas between the bottom of the cake and the pan and lift cake off of the center ring. Cool completely.