Whisk flour, cocoa powder, and salt in a medium bowl. Using electric mixer, beat butter and sugar in large bowl until fluffy.
Beat in eggs 1 at a time. Stir in dry ingredients.
Gather dough into a ball; wrap in plastic and chill at least 4 hours. (Can be made a day ahead. Keep refrigerated.).
Preheat oven to 375 degrees F. Spray 2 baking sheets with nonstick spray. Divide dough into 8 equal portions; roll into balls.
Roll 1 ball between work surface and palms into a 16 inch long rope. To form pretzel shape, lift 1 end of rope and press about 1 1/2 inches past center of rope, crossing over first loop. Transfer to baking sheet. Repeat with remaining dough.
Lightly beat egg white in small bowl to loosen. Brush pretzels with egg white. Sprinkle with raw sugar. Bake until firm, about 15 minutes. Cool on racks. (Can be made 2 days ahead. Store at room temperature in an airtight container.).
Line 2 baking sheets with parchment paper. Melt bittersweet chocolate and white chocolate separately in double boilers set over simmering water, stirring occasionally, until smooth. Dip 4 pretzels halfway into melted bittersweet chocolate; tranfer to 1 prepared baking sheet. Dip remaining 4 pretzels into white chocolate; transfer to 2nd baking sheet. Let stand at room temperature 15 minutes to allow chocolate to begin to set.
Dip for into melted bittersweet chocolate. Sweep for back and forth a few inches above white chocolate dipped pretzels, creating thin stripes, dipping fork in bittersweet chocolate as needed.
Dip another fork into melted white chocolate. Sweep for back and forth a few inches above bittersweet chocolate dipped pretzels, creating thin stripes, dipping fork in white chocolate as needed.
Let stand at room temperature until chocolate is set, about 1 hour.