Heat butter, light brown sugar, and cinnamon in small saucepan over medium heat until flowing.
Pour into greased 9" springform pan and tip to spread evenly.
Wrap outside of pan with foil to prevent leakage.
Distribute blueberries evenly across bottom of pan.
Sift flour, baking soda, and salt onto wax paper; set aside.
Cream butter, granulated and brown sugars until light and fluffy, about 2 minutes.
Add eggs, 1 at a time, mixing well and scraping bowl after each addition.
Add sifted dry ingredients, alternating with buttermilk and vanilla, beginning and ending with dry ingredients. Mix until smooth.
Pour batter on top of blueberries. Spread evenly.
Bake at 350℉ until browned and toothpick inserted in center comes out clean, 35 to 40 minutes.
Cool 10 minutes on rack.
Gently invert springform pan on cake platter. Carefully remove foil wrapping. Release ring and carefully remove. Finally, remove bottom.
Cool at least 2 more hours before serving.
Serve warm (if necessary, reheat in a 300℉ oven until warm, about 10 to 12 minutes) with ice cream, yogurt, or crème fraîche, if desired.