Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a medium bowl, whisk together the all-purpose flour, oat flour, baking powder, and salt.
In a mixing bowl, using a stand mixer or electric hand mixer, beat the eggs with the erythritol and brown sugar until creamy.
Add the slightly cool melted butter, vanilla extract, and orange zest. Mix until smooth.
Gradually add the dry ingredients to the wet ingredients, stirring until a dough forms. Fold in the dried cranberries.
Transfer the dough onto a lightly floured surface and divide in half. Shape each portion into a log, about 10-12 inches long and 2 inches wide.
Place the logs to the prepared baking sheet, leaving enough space between the logs.
Bake for 25-30 minutes, or until lightly golden around the edges.
Remove from the oven and allow the logs to cool on the baking sheet for about 15 minutes. Using a sharp knife, diagonally slice the logs into 1/2-inch thick biscotti.
Place the biscotti cut side down on the baking sheet and bake for an additional 10-12 minutes, or until they become golden and crispy.
Remove from the oven and cool completely.
In a microwave-safe bowl melt the chocolate in 20-second intervals, stirring until smooth. Drizzle the melted chocolate over the cooled biscotti or dip one end of each biscotti into the chocolate. Garnish with chopped nuts if desired.
Store the biscotti in an airtight container at room temperature for up to two weeks. They can also be frozen for up to 3 months. Enjoy!