• Pre-heat your oven to 350°F.

Prepare the Topping

• Trim the rhubarb to a length that matches the width of your cake pan or cupcake tins. Slice each stalk lengthwise into ribbons. (For mini cakes, try to cut them around 1/8 inch thick. For a full size cake, cut them about 1/4 inch thick.)
• In a large frying pan or skillet, add the sugar, lemon zest, butter and salt. Cook over medium heat, stirring frequently, until the butter has melted.
• Add the rhubarb and cook until softened by not falling apart. Remove from heat and set aside.

Make the Cake Batter

• In a large bowl, beat the butter, brown sugar and granulated sugar together until light and fluffy.
• Add the eggs one at a time, beating well after each addition.
• Stir in the vanilla.
• Add the baking powder, salt, cinnamon, ginger, cloves and nutmeg, and beat to combine.
• Add the buttermilk and stir again. The mixture will look curdled and rather thin.
• Add the flour and mix until just combined. 
• Arrange several pieces of cooked rhubarb into the bottom of each cupcake tin. Spoon a little of the juice from the cooked rhubarb into each tin as well (it's okay for it to be quite wet).
• Drop a large spoonful of cake batter into each cupcake tin on top of the rhubarb. You can spread it around a little bit with a spoon to flatten it out, but it doesn't have to be perfect. It will spread itself while it bakes. 
• Bake mini cakes for 12-15 minutes, or until a toothpick inserted into the centre comes out clean.
• Allow the cakes to cool in the pan for just a couple of minutes, before flipping them onto a wire rack to cool. (Don't let the cakes cool down too long, you want to flip them while they're still hot so they don't stick).