Heat oven to 350 degrees.
Whisk flour, baking powder and salt together in a medium bowl. Mix 6 Tablespoons butter and sugar together with an electric mixer until light and fluffy. (4 to 6 minutes on high.) Beat in eggs, vanilla and lemon zest until combined.
Turn mixer to low and add half of flour mixture, then half of the milk. Add remaining flour and milk and beat until the batter is smooth.
Melt remaining butter in a 10-inch skillet over medium heat. Add brown sugar and cook, stirring constantly, for 30 seconds or until all sugar is dissolved. Remove from heat and add enough blueberries to completely cover the bottom of the skillet.
Pour batter over the berries and smooth the top with a rubber spatula or offset spatula. Bake 35 to 40 minutes, or until cake is golden brown and a skewer inserted in the center comes out clean.
Let cool in pan for 10 minutes, then run a knife around the edge and invert on a serving plate. Serve warm or at room temperature.
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