Preheat your oven to 190°C (375°F). In a large bowl, whisk the eggs and sugar together with a mixer until the sugar dissolves. Add the softened butter and sour cream to the mixture, whisking again until combined. Sift together 2 1/2 cups flour (300g), baking powder, and a pinch of salt; gradually add this to the wet ingredients, mixing until incorporated. If the dough seems too runny, add up to 1/4 cup (50g) more flour as needed—the dough may be sticky, which is normal. Using a tablespoon, form equal-sized balls of dough, greasing your hands lightly with oil if the dough sticks. Arrange the dough balls on a parchment-lined baking sheet and bake for 15-17 minutes, until the cookies are set but not browned.

Allow the cookies to cool slightly after baking. Using a small knife or a special mold, cut a small circle from the bottom of each cookie to create a cavity for the filling. Pair the cookies by matching halves of similar size. This will make assembly neater and ensure the cookie 'sandwiches' are even.

In a bowl, whisk the dulce de leche (or boiled sweetened condensed milk) with the measured salt until smooth and creamy. If desired, you can add a spoonful of butter, a touch of mascarpone, or a bit of apricot jam for more depth of flavor. Transfer the mixture to a piping bag. Pipe the filling into the cavity of each cookie half, slightly overfilling so the filling will stick to the paired half when pressed together.

Press the paired cookie halves together gently so the filling joins them securely. Arrange the assembled cookies on a tray and place them in the fridge for about 30 minutes to allow the filling to set. I like to chill the cookies a little longer for a firmer center, but 30 minutes is usually enough.

While the cookies chill, grate the carrot and beetroot separately. Place the grated carrot and beet each in their own bowl, then use a sieve or clean cloth to strain and extract the juices into separate bowls. These vibrant juices will be used to color the cookies naturally.

Take each chilled sandwich cookie and dip one half into the carrot juice and the other half into the beet juice, letting the excess drip off. While the cookie is still wet, generously roll it in granulated sugar, ensuring even coverage. Repeat with all cookies for a playful, vibrant look. For a brighter color, I sometimes dip each half twice, letting excess juice drip off between dips.

Place the sugar-coated, colored cookies on a tray and allow them to set at room temperature for about an hour—this helps the sugar coating adhere and the colors become more pronounced. For a charming touch when serving, decorate each cookie with fresh or dried mint leaves to resemble real fruit or vegetables.