Instructions

Adjust oven rack to middle position and heat oven to 375 degrees. Line baking sheet with parchment paper. Whisk flour and baking powder together in bowl. Pulse nuts in food processor until finely ground, about 10 pulses; add to flour mixture. In now-empty food processor, pulse coconut until finely ground, about 5 pulses; add to flour-nut mixture.
Using stand mixer fitted with paddle, beat butter, sugar, and salt on medium-high speed until pale and fluffy, about 3 minutes. Add egg and yolk, lemon zest and juice, and vanilla and beat until combined. Reduce speed to low and add flour mixture in 2 additions until just combined, scraping down bowl as needed.
Using 1-tablespoon measure, scoop 12 portions of dough, roll into smooth balls, and evenly space on prepared sheet. Press dough to 1/4-inch thickness using bottom of greased measuring cup or flat-bottomed glass.
Bake until golden brown around edges, 8 to 10 minutes, rotating sheet halfway through baking. Let cookies cool on sheet for 5 minutes, then transfer to wire rack and let cool completely. Repeat twice more with remaining dough, letting sheet cool between batches.
Microwave white chocolate in bowl at 50 percent power, stirring occasionally, until melted, 2 to 3 minutes. Stir in lemon oil. Dip half of each cookie into melted white chocolate, letting excess drip back into bowl, and place on parchment-lined sheet. Let chocolate set for 30 minutes.
Microwave semisweet chocolate in bowl at 50 percent power, stirring occasionally, until melted, about 2 minutes. Dip fork into chocolate and wave back and forth over white chocolate–coated half of cookies to create lines. Let chocolate set for at least 30 minutes before serving.