In a medium bowl, whisk together flour, baking powder and kosher salt. Set aside.

In a separate large bowl, with a hand mixer (or in a standmixer fitted with a paddle attatchment) beat butter, sugar and lemon zest on medium-high until light and fluffy, 3–5.

With the mixer on low, stream in oil with mixer running. Turn off the mixer and add in eggs one at a time, beating well after each. Scrape down the sides of the bowl and mix in lemon juice, and sour cream. Mix again, then add the dry ingredients in 2 additions, mixing until just combined. Scoop half the batter into the prepared pan and smooth the top with an offset spatula. Spread an even layer of blueberry jam over the batter, then dollop the remaining batter on top — adding it in spoonfuls across the surface makes it much easier to gently spread into an even layer without disturbing the jam underneath. Bake 35–40 minutes until a toothpick comes out clean. Cool in pan 20 minutes, then turn out onto a wire rack to cool completely.

In a medium bowl, add the confectioner's sugar and the lemon juice, mix to form a thick glaze and pour on top of the cooled cake. If you wish, you can top with more lemon zest. Let the glaze set before cutting and serving.