In a separate bowl, combine flour, cake mix, baking powder and salt. Whisk to combine.
Soften butter in the microwave for 15 seconds. Combined softened butter and sugar in a mixing bowl. Beat on medium speed until light and fluffy, 2 to 3 minutes.
Add egg and vanilla extract and beat until well combined. Zest lemon into mixing bowl and beat until lemon zest is incorporated.
Reduce mixer to low speed and slowly add dry ingredients. Once combine, increase speed to medium high and beat for 1 to 2 minutes until soft dough forms.
Spoon 2 tablespoons of cookie dough onto a lined baking sheet. Gently press the top of the cookies down with the back of spoon or spatula. Bake at 350° F for 10 to12 minutes. Allow cookies to cool on the pan for several minutes before transferring to a wire rack. Cool completely.
In a small bowl, microwave white chocolate at 50% power for 30 to 60 seconds. Pour melted chocolate into a small piping bag and drizzle over cooled cookies. All chocolate to cool until firm. You can speed it up by refrigerating the cookies.