Spray 8 (4-ounce) ramekins with cooking spray. Using a 1/4 cup of sugar, coat each greased ramekin with a good sprinkle. Slice strawberries in quarters, and arrange in a spiral pattern on the bottom of each ramekin.
In a medium bowl, whisk together the flour, baking soda and salt.
In a large bowl with an electric mixer, cream together butter, mascarpone and the sugars. Beat in the eggs, 1 at a time, followed by vanilla extract. Mix in half of the flour mixture, followed by the buttermilk. Stir in the remaining flour mixture, followed by the strawberry nectar. Stir well to remove any lumps. Pour the mixture into each ramekin over the strawberries and tap the ramekins on counter to release any air bubbles. Arrange the ramekins on a half sheet tray. Bake until a toothpick inserted into the center of the mini cakes comes out clean, about 25 to 28 minutes.
Let the cakes cool for 2 minutes, then run a knife along the outside edge of the ramekin and invert the ramekins onto serving plates. Sprinkle with powdered sugar and serve.