Preheat the oven: Heat to 160 °C (320 °F) top/bottom heat, or 150 °C (300 °F) with fan. Grease and line a 24 cm round cake pan with baking paper.
Mix wet ingredients: In a large mixing bowl, whisk together the brown sugar, eggs, oil, yogurt, and vanilla until smooth and creamy.
Prepare dry ingredients: In another bowl, whisk the flour, baking powder, baking soda, cinnamon, nutmeg and salt. Sift this mixture over the wet ingredients and gently stir until just combined. Do not overmix.
Add carrots and nuts: Fold in the grated carrots and chopped pecans (or walnuts) with a spatula until evenly distributed.
Bake the cake: Pour the batter into the prepared pan and smooth the top. Bake for 50–65 minutes. Start checking at 50 minutes — the cake is ready when a toothpick inserted in the center comes out clean and the top springs back lightly when pressed.
Cool: Let the cake rest in the pan for 10 minutes, then carefully remove it and cool completely on a wire rack. (Tip: place it upside down if you’d like a flatter top.)
Make the frosting: Place mascarpone, cream cheese, heavy cream, powdered sugar, and vanilla in a bowl. Beat everything together until smooth and fluffy. Chill for 20–30 minutes before using.
Assemble & decorate: Spread the frosting evenly over the cooled cake (covering the sides too, if you like). Decorate with pecan halves arranged around the edge and finish with a light dusting of cinnamon.