Preheat oven to 350°. Line a 9-inch springform pan with parchment paper; spray edges of paper with baking spray.
In a large bowl, whisk together milk, melted butter, eggs, and vanilla bean paste. In a medium bowl, whisk together flour, sugar, baking powder, salt, and nutmeg. Gradually add flour mixture to milk 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 inside edge of pan. Spoon blackberry jam into center of ring. Pipe remaining batter on top of jam, being careful to fill any gaps. (You should not be able to see any blackberry jam.)
Bake until a wooden pick inserted in center comes out clean, 55 minutes to 1 hour. Let cool in pan for 10 minutes.
Run an of set spatula around edge of pan to loosen cake before removing from pan. Let cool completely on a wire rack. Place cake on a spinning stand or lazy Susan. Spread Buttercream Frosting on top of cake. Using a small offset spatula, create a spiral design by pressing in with the tip of the spatula and carefully spinning the stand or lazy Susan while slowly moving towards the outside edge of the cake. Garnish with sage leaves and blackberries, if desired.
Buttercream Frosting: In a large bowl, beat butter at medium speed with a mixer until creamy. Gradually add confectioners’ sugar, beating to combine. Add cream, one tablespoonful at a time, until icing is creamy and easy to spread.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/blackberry-jam-cake-recipe/