How to Make It
Preheat oven to 350°F. Line 2 rimmed baking sheets with parchment paper. Sift together flour, cocoa, instant coffee, baking powder, salt, and baking soda; set aside.
Beat sugar, eggs, and egg yolks in a large bowl with an electric mixer fitted with whisk attachment on medium-high speed until pale yellow and thick, about 5 minutes; beat in vanilla. With mixer on medium-low, gradually add butter to egg mixture, beating until well incorporated, about 2 minutes. With mixer on low, add flour mixture and beat until just combined. Add chocolate and beat to combine.
Lightly dust a work surface with cocoa powder. Divide dough in half and roll each piece of dough into a 9 x 1 1/2-inch log. Place 2 logs on each prepared baking sheet and press gently to flatten slightly.
Bake until tops begin to crack and logs are slightly firm to the touch, 18 to 20 minutes. Remove from oven and let cool on sheets about 10 minutes. Cut each log diagonally into 18 slices. Arrange slices, cut sides down, on baking sheets. Return to oven and bake, flipping cookies and rotating sheets halfway through, until dry, 18 to 20 minutes. Transfer cookies to a wire rack, and cool completely, about 20 minutes.
Place white chocolate and canola oil in a medium-sized microwave-safe bowl; microwave on HIGH, stirring every 20 seconds, until melted and smooth, about 1 minute. Dip one end of each biscotti in white chocolate, letting excess drip back into bowl; transfer to a parchment-lined sheet and set aside until dry and set, about 15 minutes.
