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).
Heat oven and prep cake pan: Preheat oven to 350 degrees. Line a 9" springform pan or a 9″ x 13″ baking pan with parchment paper and spray with baking spray (or prep with butter and flour) and set aside.
Sift dry ingredients: Sift together flour, baking powder, baking soda, and salt.
Beat butter: Beat butter, granulated sugar, and lemon zest with a mixer on medium speed in a large bowl until light and fluffy, about 2 minutes.
Add the eggs: With the mixer running, add eggs, 1 at a time, then the vanilla. Reduce speed to low.
Add in flour mixture: Add the flour mixture in 3 additions, alternating with buttermilk.
Spoon batter into pan: Spoon the batter evenly into cake pan.
Spoon on the jam: Spoon the jam in a few sections over the batter.
Swirl: With a butter knife, swirl the jam around gently.
Add on the streusel topping: Sprinkle the chilled streusel evenly over the batter
Bake the cake: Bake until cake is golden brown and a tester inserted in the center comes out clean, about (If using 9x13-inch pan it would be 30-35 minutes. If using 9-inch springform pan, about 50-55 minutes).
Cool down cake: Transfer pan to a wire rack set over a baking sheet, and let cool in pan for 15 minutes. Remove spring form (if you did round cake pan). Slice and enjoy!