Butter a 10-inch bundt pan and lightly dust the inside with flour.
Sift together and set aside the flour, baking powder and salt.
In a mixer, or by hand, cream sugar and butter until light and fluffy.
Add eggs, blending well, and then add sour cream and vanilla. Mix until combined.
Gradually fold in flour, baking powder, and salt into creamed mixture until just blended.
Reserve 1/2 cup of batter and mix with 3/4 of the jar of Wild Maine Blueberry Jam.
Pour 3/4 remaining batter into the prepared pan, smoothing to the edges and creating a slight "trough" with a spatula.
Gently spoon the blueberry mix into the batter trough, being careful not to touch the side of the pan.
Spread the remaining batter over the jam and smooth the top.
Set pan on middle rack in the oven and bake for about 50-60 minutes, or until the sides of the cake pull away from the pan and a cake tester (or toothpick) inserted in the center comes out clean.
Cool on a wire rack for at least 30 minutes. Turn onto a plate, sprinkle with confectioner's sugar, drizzle with remaining jam and enjoy!