Heat sauce pan over low heat and combine blackberries, sugar and lemon juice in the pan. Cook berries for 20-25 minutes to thicken the sauce, stirring occasionally. (Try not to crush the berries when stirring)
Once blackberry sauce has thickened, remove from heat, and let the sauce cool.
Blackberry Butter Cake
Preheat oven to 375°. Beat whipped butter and sugar with an electric mixer until smooth. Add vanilla extract and beat in eggs one at a time, making sure each egg is fully incorporated into the batter.
Next gradually add flour, scraping batter from sides of the bowl. Mix flour well until well combined but do not overmix. Place batter in the ungreased baking dish and place blackberry sauce on top of the batter. Bake for 40-45 minutes or until the tooth pick inserted in the middle of cake comes out clean.
Let cake cool for 20-25 minutes. Once cooled, cut cake into squares or use biscuit cutter to cut out small round cakes. Serve with extra blackberry sauce and dust cakes with powdered sugar.