Heat the oven to 375°F.
Butter and flour a 9-in spring form pan or a high-walled pan of similar measure, and line with parchment paper.
Wash and dry the blueberries, toss with 1 tbs of the 2c flour from the cake recipe.
If your marmalade is refrigerated, place it in a bowl and allow it to come to room temperature.
To make the Crumble, add the sugar, spices, and salt to the melted butter and whisk until the sugar has dissolved.
With a spatula, add the flour and mix into a dough. The crumble will look like grainy maple fudge.
Set the Crumble aside.
To make the Cake Batter, in a bowl, whisk the flour, baking powder, baking soda, and salt, and keep handy.
In a mixer or by hand, mix the butter and sugar until fluffy.
Add the eggs and vanilla and mix until combined.
Add 1/2 of the flour mixture and beat until combined.
Add 1/2 of the sour cream [i.e. 1/4 cup] and beat until combined.
Repeat with the remaining dry and wet ingredients. [The batter will be very thick. You're going to say, Is this cookie dough? This is never going to work! IT WILL!]
With a spatula, fold in the berries. This won't necessarily be easy. Try not to crush them.
Scoop the batter into the pan and smooth it flat with a big spatula.
In a small bowl or jar, stir the marmalade until it is smooth.
In clumps spaced evenly around the pan, add the marmalade to the cake batter by the dollop.
Gently swirl through the batter with a knife. [It won't go deep because the batter is crazy thick.]
Top the batter evenly with the chopped nuts if desired.
Sprinkle the Crumble topping over the cake, breaking it into large crumbs with your fingers. You might have to use a spoon or spatula to scrape the crumble out of the bowl.
Bake for 47-57 minutes, or until a long toothpick or butter knife in the centre comes out clean. [Watch out for burst blueberries and warm jam throwing you off!]
Cool the cake completely in the pan on a wire rack.
Remove the cake from the springform pan and carefully move cake to a serving dish. Feel free to leave the parchment on the bottom, or even the metal springform base. [If you don't use a springform pan, carefully flip the cake onto a plate. Remove the parchment. And flip back onto a serving dish.]
To make the Drizzle, add 1/2 c of the icing sugar, the vanilla and the bourbon to the melted butter, whisking to combine.
Add the citrus juice to the combined mixture and whisk to combine.
Add the additional icing sugar and whisk well until thoroughly combined and thick.
With a fork or knife toss the thick drizzle over the cake. [Alternatively, you can skip the drizzle and just dust the cake with icing sugar.]