Position rack in center of oven and preheat to 350 degrees F (180 degrees C). Butter 9-inch-diameter cake pan with 2-inch-high sides; line bottom with parchment.
Combine All-Purpose Flour (1 1/2 cup), Cornmeal (1/3 cup), Granulated Sugar (3/4 cup), Baking Powder (3 1/2 teaspoon), and Salt (1/2 teaspoon) in a large bowl; whisk to blend.
Whisk Whole Fat Kefir (1 cup), Large Egg (2), Lemon (1 tablespoon), Vanilla Extract (3/4 teaspoon) and Unsalted Butter (1/2 cup) in a small bowl.
Pour kefir mixture into the flour mixture. Using rubber spatula, gently fold liquids into flour mixture until just blended (do not stir). Scrape batter into pan; spread evenly.
Bake cake until tester inserted into center comes out clean and cake pulls away from sides of pan, about 30 minutes.
Combine Powdered Confectioners Sugar (1 1/2 cup) and Lemon (2 tablespoon) in small bowl. Stir with spoon until smooth and paste-like, adding more lemon juice by 1/2 teaspoonfuls if glaze is too thick to spread. Set aside.
Combine Fresh Blueberries (1 1/2 cup) with Brown Sugar (2/3 cup), Lemon (2/3 tablespoon), Lemon (1/6 tablespoon) in medium saucepan. Stir over medium heat until sugar dissolves and mixture comes to simmer, about 7 minutes.
Remove from heat; add remaining Fresh Blueberries (1 1/2 cup) and Salt (to taste). Using back of spoon, gently press fresh blueberries against side of pan until lightly crushed.
Immediately run knife around sides of cake. Place rack over cake in pan. Using oven mitts, hold pan and rack firmly together and invert cake onto rack. Remove pan from cake. Remove parchment paper.
Place another rack on bottom of cake; invert 1 more time so that cake is top side up. Carefully transfer to a serving platter. Stir glaze until blended. While cake is still very hot, drop glaze by tablespoonfuls onto cake; spread to within 1/2 inch of edge.
Cover with the blueberry jam and chill!