Make the streusel: Mix together flour, brown sugar, and salt. Using a pastry cutter or your fingers, cut butter into the flour mixture until small to medium clumps form. Cover, and refrigerate until ready to use (up to 3 days).

Make the cake: Cook lemon slices in a medium saucepan of simmering water for 1 minute. Drain, and repeat. Arrange lemon slices in a single layer on a parchment-lined baking sheet.

Preheat oven to 350 degrees. Butter a 9-inch angel food cake pan. Sift together flour, baking powder, baking soda, and salt. Beat butter, granulated sugar, and lemon zest with a mixer on medium speed in a large bowl until light and fluffy, about 2 minutes. With the mixer running, add eggs, 1 at a time, then the vanilla. Reduce speed to low. Add the flour mixture in 3 additions, alternating with sour cream.

Spoon 1/2 of the batter evenly into cake pan. Arrange 1/2 of the lemon slices in a single layer over the batter. Spread remaining batter evenly over the top. Cover with the remaining lemon slices in a single layer. Sprinkle the chilled streusel evenly over the batter.

Bake until cake is golden brown and a tester inserted in the center comes out clean, about 55 minutes. Transfer pan to a wire rack set over a baking sheet, and let cool in pan for 15 minutes. Run a knife around the edges of the pan, and remove outer ring. Let cool on rack for 15 minutes. Run a knife around the center tube. Slide 2 wide spatulas between the bottom of the cake and the pan, and lift cake to remove from the center tube. Let cool completely on rack.

Make the glaze: Just before serving, stir together confectioners' sugar and lemon juice in a medium bowl. Drizzle over cooled cake, letting excess drip down the sides. Let glaze set before slicing, about 5 minutes. (Cake can be stored for up to 3 days. The lemon flavor will intensify with time.).