Preheat the oven to 350ºF (177ªC). Line a large baking sheet with parchment paper or a silicone baking mat. Lightly flour the baking sheet and set aside.
In a medium size bowl, whisk together the melted butter, oil, egg, and vanilla extract until combined. Set aside.
1 Tablespoon (14g) unsalted butter, 3 Tablespoons (42mL) vegetable or canola oil, 1 large egg, 1 and ½ teaspoons vanilla extract
In a large bowl, toss together the red velvet cake mix, flour, and salt. Add the wet ingredients to the dry ingredients and gently mix with a large wooden spoon or sturdy rubber spatula until everything starts to come together. Mixture will be crumbly. This is ok. It will come together in the next step.
1 and ¼ cup (200g) red velvet cake mix, ¾ cup (90g) all-purpose flour, ⅛ teaspoon salt
Check out this Instagram reel to see how I make my biscotti. Turn dough out onto lightly floured baking sheet and, with floured hands, pat the dough into a rectangle about ½" thick. My rectangle is always 6" wide and 8" long (using a ruler or a ruled silicone baking mat is helpful here). Brush entire slab (top and sides) with egg wash.
1 large egg beaten with 1 Tablespoon water
Bake the dough slab for 18 minutes, or until the slab looks set. Remove from the oven, but do not turn off the heat. Place baking sheet on a wire rack and allow to cool for 10 minutes.
Once the dough is cool enough to touch, cut into 1" thick slices1 using a sharp knife. Cut these 1" slices in half, making a perpendicular slice down the entire slab. Set slices cut sides up on the baking sheet.
Return baking sheet to the oven to bake for 6 minutes. Remove from oven, turn biscotti over, and bake the other side for 6 minutes. Remove from the oven and allow to cool for 3 minutes on the baking sheet (cookies will become crunchy as they cool). Transfer biscotti to a wire rack to cool completely before serving. Decorate with white chocolate and sprinkles, if desired. You can also leave them plain or give a simple dusting of powdered sugar. Biscotti stay fresh in an airtight container up to 2 weeks. Baked biscotti may be frozen2 up to 3 months.
2 ounces (57g) white chocolate chips or chopped bar, festive sprinkles
Notes
Slicing the slab: I like to cut off each end of the slab so that every piece has an exposed side. See photos in the post above for an example.
Freeze biscotti: you can freeze fully baked biscotti or freeze after slicing and before baking individual biscotti on their sides. Take care moving the biscotti as they will be slightly fragile until they're fully baked.
Nutrition Disclosure
All nutritional values are approximate and provided to the reader as a courtesy. Changing ingredients and/or quantities will alter the estimated nutritional calculations.