2) Cream the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment for 4 to 5 minutes, until light. Add the eggs 1 at a time, then add the vanilla and sour cream.
3) In a separate bowl, sift together the flour, baking powder, bicarbonate of soda and salt. With the mixer on low, add the flour mixture to the batter until just combined. Finish stirring with a spatula to be sure the batter is completely mixed.
4) For the streusel, place the brown sugar, flour, cinnamon, salt and butter in a bowl and pinch together with your fingers until it forms a crumble. Mix in the walnuts, if desired.
5) Spoon half the batter into the tin and spread it out with a knife. Sprinkle with 1/2 the streusel. Spoon the rest of the batter into the tin, spread out, and scatter the remaining streusel on top. Bake for 50 to 60 minutes, until a cake tester comes out clean.
6) Cool on a wire rack for at least 30 minutes. Carefully transfer the cake onto a serving plate. Whisk the icing sugar and maple syrup together, adding a few drops of water if necessary, to make the glaze runny. Drizzle as much as you like over the cake with a fork or spoon.