FOR THE STREUSEL - in a medium bowl whisk together the flou,r sugars, salt, lemon zest and juice. Drizzle with melted butter and stir until combined. Crumble this with your fingers until you have the consistency you like. (It is quite a wet mix)
FOR THE CAKE - Place eggs, sugar and oil in a bowl of a stand mixer with the paddle attachment or use hand mixer if preferred. Mix on medium high speed until thick and pale yellow. This should take about 5 or 6 minutes. Stir in melted butter, lemon zest and juice and mix until combined. Add sifted flour mixture to the butter mixture beating until just combined. Stir in the sour cream.