Preheat oven to 350°F. Spray an 8-inch round cake pan with baking spray with flour, and line pan with parchment paper.
For cake: In a medium bowl, whisk together flour, sugar, baking powder, salt, and nutmeg. In a large bowl, whisk together milk, melted butter, eggs, and vanilla bean paste. Add milk mixture to flour mixture, stirring just until combined. Pour half of batter into prepared pan. Spoon remaining batter into a large piping bag. Pipe a ring of batter around the inside edge of the pan. Spoon Quick Blackberry-Blueberry Jam into center of the ring. Pipe remaining batter on top of jam, and smooth with an offset spatula.
Bake until a wooden pick inserted in center comes out clean, 55 minutes to 1 hour, covering with foil during last 30 minutes of baking to prevent excess browning, if necessary. Let cool in pan for 10 minutes.
For topping: In a small bowl, stir together sugar, cinnamon, and salt. Run an offset spatula around edge of pan to loosen cake before turning out. Turn cake back over (so dome is on top). Brush top and sides of cake with melted butter. Sprinkle top with sugar mixture, and press into sides of cake.
Recipe by Bake from Scratch at https://bakefromscratch.com/sugared-jam-cake/