For the streusel: Stir together the brown sugar, cinnamon and ginger in a small bowl.
For the cake: Preheat the oven to 350 degrees F. Butter, then dust twelve 6-ounce ramekins with the streusel mixture. Place the ramekins on a sheet pan.
Combine the buttermilk and vanilla extract in a liquid measuring cup. Set aside.
Combine the granulated sugar and lemon zest in the bowl of a stand mixer or a large bowl. Rub the lemon zest into the sugar. Add the flour, baking powder and salt. Beat on low speed until well blended. Add the oil with the mixer running on low speed; once that is evenly distributed, add the butter a tablespoon at a time, beating until fully incorporated. The mixture will have the consistency of coarse sand.
Add the eggs one at a time, beating well after each addition. Stop to scrape down the bowl. On medium speed, gradually add the buttermilk mixture, mixing until the batter just comes together. Do not overmix.
Spoon the batter into the prepared ramekins halfway, then sprinkle about 2 teaspoons streusel evenly on the batter in each. Spoon more batter on top to fill each ramekin three-quarters of the way. Use a table knife or skewer to gently swirl the streusel into the batter, as though you were making a marble cake. Bake until a tester inserted in the center of the ramekins comes out clean, about 30 minutes. Transfer the sheet pan to a wire rack; let the cakes cool completely.
For the quick creme fraiche: Stir the heavy cream, sour cream and salt together in a small bowl. Set aside.
For the caramel sauce: Meanwhile, mix the granulated sugar, lemon juice and salt in a deep saucepan and place on medium heat. Have a pastry brush and a small bowl of water nearby to wash the sides of the pan of any granules of sugar. When the sugar starts to brown, swirl the pan to ensure even browning. Once the sugar is amber in color, pour in the bourbon (step back, because there will be sputtering). Add the butter in 4 pieces. Swirl the pan to melt the butter, then add the heavy cream, cinnamon and ginger. Remove from the heat. The caramel will continue to thicken off the heat. Stir before serving.
Turn the cakes out of the ramekins. To serve, place a cake on a serving plate. Spoon the caramel over the cake with a dollop of "creme fraiche." Garnish with the crystallized ginger.