Preheat the oven to 350F degrees and adjust the oven rack to the middle position. Lightly grease and flour 5 cups of the brownie pan or muffin pan. Cut 1-inch strips of parchment paper - long enough to overhang the cups - and place in the middle of the cups.
In a liquid measuring cup, add the milk and sour cream and mix until combined.
In a medium mixing bowl, add the butter and sugar. Use a hand-held mixer on medium speed to beat together until creamy. Add the lemon juice, lemon zest, vanilla extract and egg. Beat until combined.
In a small mixing bowl, sift together the flour, baking powder, and salt. Add 1/3 of the dry mix to the wet ingredients and beat on low speed until combined. Add 1/2 of the milk mix, and beat until just combined. Add another 1/3 of the dry mix, beat until just combined. Then the remaining milk mix, beat, followed by the last of the dry mix. I like to fold in the last of the dry mix with a spatula, until no flour streaks remain.
Pour the batter evenly into the prepared baking pan cups. Bake for 15 minutes, or until a toothpick inserted in the middle of the cakes comes out clean.
Cool the mini cakes in the baking pan for 5 minutes, then gently lift out of the cups using the parchment paper strips. Cool completely on a wire rack before frosting and serving.